The XMLHttpRequest Level 2 standard (still a working draft) defines the FormData interface. This interface enables appending File objects to XHR-requests (Ajax-requests).

Btw, this is a new feature – in the past, the “hidden-iframe-trick” was used (read about that in my other question).

This is how it works (example):

var xhr = new XMLHttpRequest(),
    fd = new FormData();

fd.append( 'file', input.files[0] );
xhr.open( 'POST', 'http://example.com/script.php', true );
xhr.onreadystatechange = handler;
xhr.send( fd );

where input is a <input type="file"> field, and handler is the success-handler for the Ajax-request.

This works beautifully in all browsers (again, except IE).

Now, I would like to make this functionality work with jQuery. I tried this:

var fd = new FormData();    
fd.append( 'file', input.files[0] );

$.post( 'http://example.com/script.php', fd, handler );

Unfortunately, that won’t work (an “Illegal invocation” error is thrown – screenshot is here). I assume jQuery expects a simple key-value object representing form-field-names / values, and the FormData instance that I’m passing in is apparently incompatible.

Now, since it is possible to pass a FormData instance into xhr.send(), I hope that it is also possible to make it work with jQuery.

---

Update:

I’ve created a “feature ticket” over at jQuery’s Bug Tracker. It’s here: http://bugs.jquery.com/ticket/9995

I was suggested to use an “Ajax prefilter”…

---

Update:

First, let me give a demo demonstrating what behavior I would like to achieve.

HTML:

<form>
    <input type="file" id="file" name="file">
    <input type="submit">
</form>

JavaScript:

$( 'form' ).submit(function ( e ) {
    var data, xhr;

    data = new FormData();
    data.append( 'file', $( '#file' )[0].files[0] );

    xhr = new XMLHttpRequest();

    xhr.open( 'POST', 'http://hacheck.tel.fer.hr/xml.pl', true );
    xhr.onreadystatechange = function ( response ) {};
    xhr.send( data );

    e.preventDefault();
});

The above code results in this HTTP-request:

This is what I need – I want that “multipart/form-data” content-type!

---

The proposed solution would be like so:

$( 'form' ).submit(function ( e ) {
    var data;

    data = new FormData();
    data.append( 'file', $( '#file' )[0].files[0] );

    $.ajax({
        url: 'http://hacheck.tel.fer.hr/xml.pl',
        data: data,
        processData: false,
        type: 'POST',
        success: function ( data ) {
            alert( data );
        }
    });

    e.preventDefault();
});

However, this results in:

As you can see, the content type is wrong…