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how to upload separately selected files in form POST request? [duplicate]

This question already has answers here:
Uploading multiple files using formData()
(18 answers)
Closed 5 years ago.
What I want
People click add image button, they select an image, image is added to gallery.
They can delete images by clicking cross sign and re click add image button to add more images.
This all works, I've a reference to all File elements.
However, I can't figure out how to send the files in post request of form.
Problem the problem is that You can't create FileList out array of file, or set array of files as input.files = arrOfFiles.
Input element itself doesn't let you add more files, or remove files... it simply replaces old file with new file(s).
which is not what i want therefore i'm keep reference to file objects in js, and letting user remove images or add more.
I know i can send individual file as XHR, but I want to send them through form that already exists.
I wanted to know a way to send files through form not js, but apparently that's not possible

That's exactly what the FormData API is for: create from scratch a form's data that you will be able to upload to your server as if it were created from a <form> object, except that you can control what goes there or not.
So to append a File or a Blob in a FormData, the code is
var fd = new FormData();
fd.append(field_name, blob, file_name);
To append multiple files, you can call again fd.append, but note that backend often need to have the field_name formatted in such a way they can know multiple values are expected here.
Usually this is done by adding [] after your fieldname.
var fd = new FormData();
fd.append('files[]', blob_1, file_name_1);
fd.append('files[]', blob_2, file_name_2);
And then you can send it through an AJAX request to your server, which won't make the difference between this request and a real one made froma single <input multiple name="files[]">.
Note that in case of File, file_name is optional and will default to the File's name if not set. However, it is needed for Blobs if you don't want a random name to be set.
var file_1 = new File(['foo'], 'file1.txt',{type:'text/plain'});
var file_2 = new File(['bar'], 'file2.txt', {type:'text/plain'});
var fd = new FormData();
fd.append('files[]', file_1);
fd.append('files[]', file_2);
console.log(...fd.entries());
// and to send it to your server
var xhr = new XMLHttpRequest();
xhr.open('POST', 'your_server_url');
xhr.send(fd);

If you are just going to upload images. Consider using base64 encoding. That is what I did. Assign base64 encoded URL to stringify JSON, or whatever you wish that is easier for you.
When the data is POST-ed, do the conversion there, to get back your files for saving them. Decoding base64 will be dependent on your backend language though of which you did not mention in your question.
In JavaScript, you can use this function to load up the image in preview state like you mentioned.
function previewImage (inputId, eleId) {
if (window.FileReader) {
var oPreviewImg = null, oFReader = new window.FileReader(),
rFilter = /^(?:image\/bmp|image\/cis\-cod|image\/gif|image\/ief|image\/jpeg|image\/jpeg|image\/jpeg|image\/pipeg|image\/png|image\/svg\+xml|image\/tiff|image\/x\-cmu\-raster|image\/x\-cmx|image\/x\-icon|image\/x\-portable\-anymap|image\/x\-portable\-bitmap|image\/x\-portable\-graymap|image\/x\-portable\-pixmap|image\/x\-rgb|image\/x\-xbitmap|image\/x\-xpixmap|image\/x\-xwindowdump)$/i;
oFReader.onload = function (oFREvent) {
if (!oPreviewImg) {
var newPreview = document.getElementById(eleId);
oPreviewImg = new Image();
oPreviewImg.style.width = (newPreview.offsetWidth).toString() + "px";
oPreviewImg.style.height = (newPreview.offsetHeight).toString() + "px";
if(newPreview.children.length > 0)
newPreview.replaceChild(oPreviewImg, newPreview.children[0]);
else
newPreview.appendChild(oPreviewImg);
}
oPreviewImg.src = oFREvent.target.result;
// Add Bootstrap's img-thumbnail class to the image frame
oPreviewImg.classList.add("img-thumbnail");
};
return function () {
var aFiles = document.getElementById(inputId).files;
if (aFiles.length === 0) { return; }
if (!rFilter.test(aFiles[0].type)) { alert("You must select a valid image file!"); return; }
oFReader.readAsDataURL(aFiles[0]);
}
}
if (navigator.appName === "Microsoft Internet Explorer") {
return function () {
document.getElementById(eleId).filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = document.getElementById(inputId).value;
}
}
};
Then you can extract the base64 URL from the src attribute of the img element.
You may come out with your own preferred method of "cancelling" your upload by maybe destroying the img element.

Save canvas element to file [duplicate]

Having an HTML Form that is submittet via POST (user clicking the
submit button).
Furthermore having an image that is read via Javascript of a canvas
object (getImageData()).
Question:
How to "inject" this image data into the HTML Form, so that it gets uploaded as Content-Type:multipart/form-data and can be processed via the existing PHP Frameworks Data Extraction Logic?
Example from a <input type="file" upload captured with CHrome in a POST request => it should look like this
------WebKitFormBoundaryBBAQ5B4Ax1NgxFmD
Content-Disposition: form-data; name="images"; filename="fooimage.png"
Content-Type: image/png
Problem:
I know how to uploed it in a seperate request (via ajax, seperate from the form). I know how to upload it as base64 Data an process it manually in the form.
But I do not know how to send the Image Data along the exiting Form so that it looks for the PHP Serverside Scripts exactly the same as an image that is send via <input type="file"...
Reason: Symphony (FileUpload Object) checks if a file is uploaded via the POST Form and fails if I manulally instanciate the object with the data.
So the best solution would be (in regards to a lot of other things, like testing, avoiding unnecessary logik), if the data would be passed the same as a regular form upload. How to do this?
Thanks!

You can use a FormData object to get the values of your form, and then append a blob version of your canvas into the FormData.
This blob will be seen as a file by the server.
Unfortunately, all browsers still don't support the native canvas.toBlob() method, and even worth, all implementations are not the same.
All major browsers now support the toBlob method, and you can find a polyfill on mdn for older browsers.
// the function to create and send our FormData
var send = function(form, url, canvas, filename, type, quality, callback) {
canvas.toBlob(function(blob){
var formData = form ? new FormData(form) : new FormData();
formData.append('file', blob, filename);
var xhr = new XMLHttpRequest();
xhr.onload = callback;
xhr.open('POST', url);
xhr.send(formData);
}, type, quality);
};
// How to use it //
var form = document.querySelector('form'), // the form to construct the FormData, can be null or undefined to send only the image
url = 'http://example.com/upload.php', // required, the url where we'll send it
canvas = document.querySelector('canvas'), // required, the canvas to send
filename = (new Date()).getTime() + '.jpg',// required, a filename
type = 'image/jpeg', // optional, the algorithm to encode the canvas. If omitted defaults to 'image/png'
quality = .5, // optional, if the type is set to jpeg, 0-1 parameter that sets the quality of the encoding
callback = function(e) {console.log(this.response);}; // optional, a callback once the xhr finished
send(form, url, canvas, filename, type, quality, callback);
PHP side would then be :
if ( isset( $_FILES["file"] ) ){
$dir = 'some/dir/';
$blob = file_get_contents($_FILES["file"]['tmp_name']);
file_put_contents($dir.$_FILES["file"]["name"], $blob);
}

Simulate XHR2 file upload

I have a HTML upload button to send (multiple) files to a server which responds with JSON. Based on that response, my application flow continues.
Now, to test the rest of my code (dependent on the server response), I would like to simulate the file upload so that I do not have to manually select and upload new files on every reload.
Following is a simplified version of my upload method:
uploadToServer: function (file) {
var self = this,
data = new FormData(),
xhr = new XMLHttpRequest();
// When the request has successfully completed.
xhr.onload = function () {
var response = $.parseJSON(this.responseText);
var photo = new App.Models.Photo({
filename: response.filename,
name: response.uuid
});
var photoView = new App.Views.Photo({ model: photo });
$('.photos').append(photoView.render().el);
};
// Send to server, where we can then access it with $_FILES['file].
data.append('file', file);
xhr.open('POST', this.url);
xhr.send(data);
}
The uploadToServer parameter file is a File-object from FileList. And as you can see, my server awaits the file inside of $_FILES['file'].
It would be awesome if a could simulate a real File-object being passed into uploadToServer, since I then do not have to worry about my existing events such as xhr.onload and xhr.onprogress being available.
If that won't be possible, I could create a custom uploadToServer method and send a regular AJAX-request and fake-respond on my server. But how can I then use and bind the code from my events (xhr.onload, xhr.onprogress etc.) to that AJAX-request?

You could simulate a file upload by creating a canvas, converting it to a file and passing the result to your method.
Create a canvas : here, a nice red square
var canvas = document.createElement('canvas'),
ctx = canvas.getContext("2d");
canvas.width = 100;
canvas.height = 100;
ctx.fillStyle = '#ff0000';
ctx.fillRect(0, 0, 100, 100);
document.body.appendChild(canvas);
Convert it to a file and pass it to your method: I used Canvas to Blob to simplify the test but you probably can extract the bare bones to fit your needs.
canvas.toBlob( function (blob) {
m.uploadToServer(blob);
}, 'image/jpeg');
And a Fiddle http://jsfiddle.net/pvWxx/ Be sure to open a console to see the events and the request.
If you want to pass a filename with your blob, you can pass a third argument to formData.append, see How to give a Blob uploaded as FormData a file name? and an updated Fiddle http://jsfiddle.net/pvWxx/1/

Ajax in .NET doesn't want to to load images as it should

Every way I try to obtain what I want with .NET and MSIE 8 desperately fails: Since I coulnd't manage to preload images with "GET", i decided to get them in base64: They came to the client side all right, but there I discovered MSIE8 can't manage base64 over 32Kb, so I'm down again.
So I came back to the classic method I found several times on the web, and there it goes:
var img = new Image();
var ajax = new ActiveXObject('Microsoft.XMLHTTP');
ajax.open("GET", url, false)
ajax.send(null);
var res = ajax.status;
if (res == 200) // succès
{
// this program crashes on next line as soon as 'url' points a jpg file
var tx = rajax.responseText;
// this program crashes on previous line except if 'url' points a text file
img.src = tx;
}
So what can I do to be able to load my image this way ? thanks for your help.

You're misunderstanding the src property.
src takes a URL, not an actual image.
You don't need any AJAX.

JavaScript/jQuery to download file via POST with JSON data

I have a jquery-based single-page webapp. It communicates with a RESTful web service via AJAX calls.
I'm trying to accomplish the following:
Submit a POST that contains JSON data to a REST url.
If the request specifies a JSON response, then JSON is returned.
If the request specifies a PDF/XLS/etc response, then a downloadable binary is returned.
I have 1 & 2 working now, and the client jquery app displays the returned data in the web page by creating DOM elements based on the JSON data. I also have #3 working from the web-service point of view, meaning it will create and return a binary file if given the correct JSON parameters. But I'm unsure the best way to deal with #3 in the client javascript code.
Is it possible to get a downloadable file back from an ajax call like this? How do I get the browser to download and save the file?
$.ajax({
type: "POST",
url: "/services/test",
contentType: "application/json",
data: JSON.stringify({category: 42, sort: 3, type: "pdf"}),
dataType: "json",
success: function(json, status){
if (status != "success") {
log("Error loading data");
return;
}
log("Data loaded!");
},
error: function(result, status, err) {
log("Error loading data");
return;
}
});
The server responds with the following headers:
Content-Disposition:attachment; filename=export-1282022272283.pdf
Content-Length:5120
Content-Type:application/pdf
Server:Jetty(6.1.11)
Another idea is to generate the PDF and store it on the server and return JSON that includes a URL to the file. Then, issue another call in the ajax success handler to do something like the following:
success: function(json,status) {
window.location.href = json.url;
}
But doing that means I would need to make more than one call to the server, and my server would need to build downloadable files, store them somewhere, then periodically clean up that storage area.
There must be a simpler way to accomplish this. Ideas?
EDIT: After reviewing the docs for $.ajax, I see that the response dataType can only be one of xml, html, script, json, jsonp, text, so I'm guessing there is no way to directly download a file using an ajax request, unless I embed the binary file in using Data URI scheme as suggested in the #VinayC answer (which is not something I want to do).
So I guess my options are:
Not use ajax and instead submit a form post and embed my JSON data into the form values. Would probably need to mess with hidden iframes and such.
Not use ajax and instead convert my JSON data into a query string to build a standard GET request and set window.location.href to this URL. May need to use event.preventDefault() in my click handler to keep browser from changing from the application URL.
Use my other idea above, but enhanced with suggestions from the #naikus answer. Submit AJAX request with some parameter that lets web-service know this is being called via an ajax call. If the web service is called from an ajax call, simply return JSON with a URL to the generated resource. If the resource is called directly, then return the actual binary file.
The more I think about it, the more I like the last option. This way I can get information back about the request (time to generate, size of file, error messages, etc.) and I can act on that information before starting the download. The downside is extra file management on the server.
Any other ways to accomplish this? Any pros/cons to these methods I should be aware of?

letronje's solution only works for very simple pages. document.body.innerHTML += takes the HTML text of the body, appends the iframe HTML, and sets the innerHTML of the page to that string. This will wipe out any event bindings your page has, amongst other things. Create an element and use appendChild instead.
$.post('/create_binary_file.php', postData, function(retData) {
var iframe = document.createElement("iframe");
iframe.setAttribute("src", retData.url);
iframe.setAttribute("style", "display: none");
document.body.appendChild(iframe);
});
Or using jQuery
$.post('/create_binary_file.php', postData, function(retData) {
$("body").append("<iframe src='" + retData.url+ "' style='display: none;' ></iframe>");
});
What this actually does: perform a post to /create_binary_file.php with the data in the variable postData; if that post completes successfully, add a new iframe to the body of the page. The assumption is that the response from /create_binary_file.php will include a value 'url', which is the URL that the generated PDF/XLS/etc file can be downloaded from. Adding an iframe to the page that references that URL will result in the browser promoting the user to download the file, assuming that the web server has the appropriate mime type configuration.

I've been playing around with another option that uses blobs. I've managed to get it to download text documents, and I've downloaded PDF's (However they are corrupted).
Using the blob API you will be able to do the following:
$.post(/*...*/,function (result)
{
var blob=new Blob([result]);
var link=document.createElement('a');
link.href=window.URL.createObjectURL(blob);
link.download="myFileName.txt";
link.click();
});
This is IE 10+, Chrome 8+, FF 4+. See https://developer.mozilla.org/en-US/docs/Web/API/URL.createObjectURL
It will only download the file in Chrome, Firefox and Opera. This uses a download attribute on the anchor tag to force the browser to download it.

I know this kind of old, but I think I have come up with a more elegant solution. I had the exact same problem. The issue I was having with the solutions suggested were that they all required the file being saved on the server, but I did not want to save the files on the server, because it introduced other problems (security: the file could then be accessed by non-authenticated users, cleanup: how and when do you get rid of the files). And like you, my data was complex, nested JSON objects that would be hard to put into a form.
What I did was create two server functions. The first validated the data. If there was an error, it would be returned. If it was not an error, I returned all of the parameters serialized/encoded as a base64 string. Then, on the client, I have a form that has only one hidden input and posts to a second server function. I set the hidden input to the base64 string and submit the format. The second server function decodes/deserializes the parameters and generates the file. The form could submit to a new window or an iframe on the page and the file will open up.
There's a little bit more work involved, and perhaps a little bit more processing, but overall, I felt much better with this solution.
Code is in C#/MVC
public JsonResult Validate(int reportId, string format, ReportParamModel[] parameters)
{
// TODO: do validation
if (valid)
{
GenerateParams generateParams = new GenerateParams(reportId, format, parameters);
string data = new EntityBase64Converter<GenerateParams>().ToBase64(generateParams);
return Json(new { State = "Success", Data = data });
}
return Json(new { State = "Error", Data = "Error message" });
}
public ActionResult Generate(string data)
{
GenerateParams generateParams = new EntityBase64Converter<GenerateParams>().ToEntity(data);
// TODO: Generate file
return File(bytes, mimeType);
}
on the client
function generate(reportId, format, parameters)
{
var data = {
reportId: reportId,
format: format,
params: params
};
$.ajax(
{
url: "/Validate",
type: 'POST',
data: JSON.stringify(data),
dataType: 'json',
contentType: 'application/json; charset=utf-8',
success: generateComplete
});
}
function generateComplete(result)
{
if (result.State == "Success")
{
// this could/should already be set in the HTML
formGenerate.action = "/Generate";
formGenerate.target = iframeFile;
hidData = result.Data;
formGenerate.submit();
}
else
// TODO: display error messages
}

There is a simplier way, create a form and post it, this runs the risk of resetting the page if the return mime type is something that a browser would open, but for csv and such it's perfect
Example requires underscore and jquery
var postData = {
filename:filename,
filecontent:filecontent
};
var fakeFormHtmlFragment = "<form style='display: none;' method='POST' action='"+SAVEAS_PHP_MODE_URL+"'>";
_.each(postData, function(postValue, postKey){
var escapedKey = postKey.replace("\\", "\\\\").replace("'", "\'");
var escapedValue = postValue.replace("\\", "\\\\").replace("'", "\'");
fakeFormHtmlFragment += "<input type='hidden' name='"+escapedKey+"' value='"+escapedValue+"'>";
});
fakeFormHtmlFragment += "</form>";
$fakeFormDom = $(fakeFormHtmlFragment);
$("body").append($fakeFormDom);
$fakeFormDom.submit();
For things like html, text and such, make sure the mimetype is some thing like application/octet-stream
php code
<?php
/**
* get HTTP POST variable which is a string ?foo=bar
* #param string $param
* #param bool $required
* #return string
*/
function getHTTPPostString ($param, $required = false) {
if(!isset($_POST[$param])) {
if($required) {
echo "required POST param '$param' missing";
exit 1;
} else {
return "";
}
}
return trim($_POST[$param]);
}
$filename = getHTTPPostString("filename", true);
$filecontent = getHTTPPostString("filecontent", true);
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"$filename\"");
echo $filecontent;

It is been a while since this question was asked but I had the same challenge and want to share my solution. It uses elements from the other answers but I wasn't able to find it in its entirety. It doesn't use a form or an iframe but it does require a post/get request pair. Instead of saving the file between the requests, it saves the post data. It seems to be both simple and effective.
client
var apples = new Array();
// construct data - replace with your own
$.ajax({
type: "POST",
url: '/Home/Download',
data: JSON.stringify(apples),
contentType: "application/json",
dataType: "text",
success: function (data) {
var url = '/Home/Download?id=' + data;
window.location = url;
});
});
server
[HttpPost]
// called first
public ActionResult Download(Apple[] apples)
{
string json = new JavaScriptSerializer().Serialize(apples);
string id = Guid.NewGuid().ToString();
string path = Server.MapPath(string.Format("~/temp/{0}.json", id));
System.IO.File.WriteAllText(path, json);
return Content(id);
}
// called next
public ActionResult Download(string id)
{
string path = Server.MapPath(string.Format("~/temp/{0}.json", id));
string json = System.IO.File.ReadAllText(path);
System.IO.File.Delete(path);
Apple[] apples = new JavaScriptSerializer().Deserialize<Apple[]>(json);
// work with apples to build your file in memory
byte[] file = createPdf(apples);
Response.AddHeader("Content-Disposition", "attachment; filename=juicy.pdf");
return File(file, "application/pdf");
}

In short, there is no simpler way. You need to make another server request to show PDF file. Al though, there are few alternatives but they are not perfect and won't work on all browsers:
Look at data URI scheme. If binary data is small then you can perhaps use javascript to open window passing data in URI.
Windows/IE only solution would be to have .NET control or FileSystemObject to save the data on local file system and open it from there.

Not entirely an answer to the original post, but a quick and dirty solution for posting a json-object to the server and dynamically generating a download.
Client side jQuery:
var download = function(resource, payload) {
$("#downloadFormPoster").remove();
$("<div id='downloadFormPoster' style='display: none;'><iframe name='downloadFormPosterIframe'></iframe></div>").appendTo('body');
$("<form action='" + resource + "' target='downloadFormPosterIframe' method='post'>" +
"<input type='hidden' name='jsonstring' value='" + JSON.stringify(payload) + "'/>" +
"</form>")
.appendTo("#downloadFormPoster")
.submit();
}
..and then decoding the json-string at the serverside and setting headers for download (PHP example):
$request = json_decode($_POST['jsonstring']), true);
header('Content-Type: application/csv');
header('Content-Disposition: attachment; filename=export.csv');
header('Pragma: no-cache');

$scope.downloadSearchAsCSV = function(httpOptions) {
var httpOptions = _.extend({
method: 'POST',
url: '',
data: null
}, httpOptions);
$http(httpOptions).then(function(response) {
if( response.status >= 400 ) {
alert(response.status + " - Server Error \nUnable to download CSV from POST\n" + JSON.stringify(httpOptions.data));
} else {
$scope.downloadResponseAsCSVFile(response)
}
})
};
/**
* #source: https://github.com/asafdav/ng-csv/blob/master/src/ng-csv/directives/ng-csv.js
* #param response
*/
$scope.downloadResponseAsCSVFile = function(response) {
var charset = "utf-8";
var filename = "search_results.csv";
var blob = new Blob([response.data], {
type: "text/csv;charset="+ charset + ";"
});
if (window.navigator.msSaveOrOpenBlob) {
navigator.msSaveBlob(blob, filename); // #untested
} else {
var downloadContainer = angular.element('<div data-tap-disabled="true"><a></a></div>');
var downloadLink = angular.element(downloadContainer.children()[0]);
downloadLink.attr('href', window.URL.createObjectURL(blob));
downloadLink.attr('download', "search_results.csv");
downloadLink.attr('target', '_blank');
$document.find('body').append(downloadContainer);
$timeout(function() {
downloadLink[0].click();
downloadLink.remove();
}, null);
}
//// Gets blocked by Chrome popup-blocker
//var csv_window = window.open("","","");
//csv_window.document.write('<meta name="content-type" content="text/csv">');
//csv_window.document.write('<meta name="content-disposition" content="attachment; filename=data.csv"> ');
//csv_window.document.write(response.data);
};

I think the best approach is to use a combination, Your second approach seems to be an elegant solution where browsers are involved.
So depending on the how the call is made. (whether its a browser or a web service call) you can use a combination of the two, with sending a URL to the browser and sending raw data to any other web service client.

Found it somewhere long time ago and it works perfectly!
let payload = {
key: "val",
key2: "val2"
};
let url = "path/to/api.php";
let form = $('<form>', {'method': 'POST', 'action': url}).hide();
$.each(payload, (k, v) => form.append($('<input>', {'type': 'hidden', 'name': k, 'value': v})) );
$('body').append(form);
form.submit();
form.remove();

I have been awake for two days now trying to figure out how to download a file using jquery with ajax call. All the support i got could not help my situation until i try this.
Client Side
function exportStaffCSV(t) {
var postData = { checkOne: t };
$.ajax({
type: "POST",
url: "/Admin/Staff/exportStaffAsCSV",
data: postData,
success: function (data) {
SuccessMessage("file download will start in few second..");
var url = '/Admin/Staff/DownloadCSV?data=' + data;
window.location = url;
},
traditional: true,
error: function (xhr, status, p3, p4) {
var err = "Error " + " " + status + " " + p3 + " " + p4;
if (xhr.responseText && xhr.responseText[0] == "{")
err = JSON.parse(xhr.responseText).Message;
ErrorMessage(err);
}
});
}
Server Side
[HttpPost]
public string exportStaffAsCSV(IEnumerable<string> checkOne)
{
StringWriter sw = new StringWriter();
try
{
var data = _db.staffInfoes.Where(t => checkOne.Contains(t.staffID)).ToList();
sw.WriteLine("\"First Name\",\"Last Name\",\"Other Name\",\"Phone Number\",\"Email Address\",\"Contact Address\",\"Date of Joining\"");
foreach (var item in data)
{
sw.WriteLine(string.Format("\"{0}\",\"{1}\",\"{2}\",\"{3}\",\"{4}\",\"{5}\",\"{6}\"",
item.firstName,
item.lastName,
item.otherName,
item.phone,
item.email,
item.contact_Address,
item.doj
));
}
}
catch (Exception e)
{
}
return sw.ToString();
}
//On ajax success request, it will be redirected to this method as a Get verb request with the returned date(string)
public FileContentResult DownloadCSV(string data)
{
return File(new System.Text.UTF8Encoding().GetBytes(data), System.Net.Mime.MediaTypeNames.Application.Octet, filename);
//this method will now return the file for download or open.
}
Good luck.

I liked Frank's idea and decided to do my own twist to it. As trying to do it in one post is very complicated, I'm using the two post method but only hitting the database once and no need to save the file or clean up file when completed.
First I run the ajax request to retrieve the data but instead of returning the data from the controller I will return a GUID that is tied to a TempData storage of the records.
$.get("RetrieveData", { name: "myParam"} , function(results){
window.location = "downloadFile?id=" + results
});
public string RetrieveData(string name)
{
var data = repository.GetData(name);
string id = Guid.NewGuid().ToString();
var file = new KeyValuePair<string, MyDataModel>(name, data);
TempData[id]=file;
return id;
}
Then when I call the window.location I pass the Guid to the new method and get the data from TempData. After this method is executed TempData will be free.
public ActionResult DownloadFile(string id)
{
var file = (KeyValuePair<string,MyDataModel>)TempData[id];
var filename = file.Key;
var data = file.Value;
var byteArray = Encoding.UTF8.GetBytes(data);
...
return File(byteArray, "text/csv", "myFile.csv");
}

Another approach instead of saving the file on the server and retrieving it, is to use .NET 4.0+ ObjectCache with a short expiration until the second Action (at which time it can be definitively dumped). The reason that I want to use JQuery Ajax to do the call, is that it is asynchronous. Building my dynamic PDF file takes quite a bit of time, and I display a busy spinner dialog during that time (it also allows other work to be done). The approach of using the data returned in the "success:" to create a Blob does not work reliably. It depends on the content of the PDF file. It is easily corrupted by data in the response, if it is not completely textual which is all that Ajax can handle.

Solution
Content-Disposition attachment seems to work for me:
self.set_header("Content-Type", "application/json")
self.set_header("Content-Disposition", 'attachment; filename=learned_data.json')
Workaround
application/octet-stream
I had something similar happening to me with a JSON, for me on the server side I was setting the header to
self.set_header("Content-Type", "application/json")
however when i changed it to:
self.set_header("Content-Type", "application/octet-stream")
It automatically downloaded it.
Also know that in order for the file to still keep the .json suffix you will need to it on filename header:
self.set_header("Content-Disposition", 'filename=learned_data.json')

The Problems with Making your own events
Many of the solutions proposed on this article have the JavaScript run asynchronously and create a link element then calling
const a = documet.createElement("a")
a.click()
or creating a mouse event
new MouseEvent({/* ...some config */})
This would seem fine right? What could be wrong with this?
What is an Event-Sourcing?
Event sourcing has a bunch of meanings across computing such as a system of pub sub in a cloud based architecture, or the browser api EventSource. In the context of a browser
all events have a source and that source has hidden property that says who initiated this event (the user or the site).
Knowing this we can start to understand why two click events might not be treated the same
user click* new MouseEvent()
----------- -----------
| Event 1 | | Event 2 |
----------- -----------
| |
|----------------------|
|
|
----------------------
| Permissions Policy | Available in chrome allows the server to control
---------------------- what features are going to be used by the JS
|
|
----------------------------
| Browser Fraud Protection | The Browser REALLY doesnt like being told to pretend
---------------------------- to be a user. If you will remember back to the early
| 2000s when one click spun off 2000 pop ups. Well here
| is where popups are blocked, fraudulent ad clicks are
\ / thrown out, and most importantly for our case stops
v fishy downloads
JavaScript Event Fires
So I just Can't Download off A POST That's Dumb
No, of course you can. You just need to give the user a chance to create the event. Here are a number of patterns that you can use to create user flows that are obvious and convectional and will not be flagged as fraud. (using jsx sorry not sorry)
A Form can be used to navigate to a url with a post action.
const example = () => (
<form
method="POST"
action="/super-api/stuff"
onSubmit={(e) => {/* mutably change e form data but don't e.preventDetfault() */}}
>
{/* relevant input fields of your download */}
</form>
)
Preloading If your download is non-configurable you may want to consider preloading the download into resp.blob() or new Blob(resp) this tells the browser that this is a file and we wont be doing any string operations on it. As with the other answers you can use window.URL.createObjectURL what is not mentioned is that
createObjectURL CAN MAKE A MEMORY LEAK IN JAVASCRIPTsource
If you don't want the C++ bully's to come make fun of you you must free this memory. Ahh but I'm just a hobbiest who loves his garbage collector. Have no fear this is very simple if you are working in most frameworks (for me react) you just register some sort of clean up effect on your component and your right as rain.
const preload = () => {
const [payload, setPayload] = useState("")
useEffect(() => {
fetch("/super-api/stuff")
.then((f) => f.blob())
.then(window.URL.createObjectURL)
.then(setPayload)
return () => window.URL.revokeObjectURL(payload)
}, [])
return (<a href={payload} download disabled={payload === ""}>Download Me</a>)
}

I think I got close, but something is corrupting the file (Image), any way, maybe some one can disclose the problem of this approach
$.ajax({
url: '/GenerateImageFile',
type: 'POST',
cache: false,
data: obj,
dataType: "text",
success: function (data, status, xhr) {
let blob = new Blob([data], { type: "image/jpeg" });
let a = document.createElement('a');
a.href = window.URL.createObjectURL(blob);
a.download = "test.jpg";
document.body.appendChild(a);
a.click();
document.body.removeChild(a);
window.URL.removeObjectURL(a.href);
},
complete: function () {
},
beforeSend: function () {
}
});

With HTML5, you can just create an anchor and click on it. There is no need to add it to the document as a child.
const a = document.createElement('a');
a.download = '';
a.href = urlForPdfFile;
a.click();
All done.
If you want to have a special name for the download, just pass it in the download attribute:
const a = document.createElement('a');
a.download = 'my-special-name.pdf';
a.href = urlForPdfFile;
a.click();