Jquery ajax returning 404 not found

It seemed to be a problem with the FormData object. Once I changed my method to use .serialize() instead, the page worked just fine.

$("form#applyform").submit(function(){

    var data = $("form#applyform").serialize();
    jQuery.each($('#file')[0].files, function(i, file) {
        data.append('file-'+i, file);
    });

    $.ajax({
        url: 'ValidateApplication.php',
        data: data,
        cache: false,
        contentType: false,
        processData: false,
        type: 'POST',
        success: function(data){
            alert(data);
        }
    });
}

I've had the same issue and after 2 hours looking for what was causing the 404 Not Found error I found that I was recently playing with the header() from PHP and had forgotten to delete the following line of code:

header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found"); 

After deleting it, my Ajax functions became normal again.

For me, it was that I used an input field with name="name" which made the called page return a 404. Weird stuff, hope this helps someone.