Safari Xhr Upload Stucks (sometimes)

I build a upload script using jquery-File-Upload. Now I have the weird behavior that the upload stucks from time to time (let's say every eighth time). This only happens in Safari

Solution 1:

Turns out that this solution by Ben fixes the issue.

In case you use jquery-File-Upload it looks like this (caution: i'm using the v4 version):

$(function () {
        $('#file_upload').fileUploadUI({
            //some other codebeforeSend: function (event, files, index, xhr, handler, callBack) {
                $.ajax({
                    async: false,
                    url: 'closeconnection.php'
                });
                callBack();
            }
        });
    });

Where the corresponding php script looks like this:

<?php
    header("Connection: Close");
?>

WTF??

Does this has something to do with Safari caching? Or with still open POST requests? Let me know if you have any idea...