In my experience with latest release of X3 i found issue or bug ( You can decide about it ) with uploader.
When i upload mp4 file with average size 30-50 M the uploader is works just fine.
When i try to upload files about 120 - 150 M the uploader working strange - it start to upload, then, when about 8-9% is done it's start upload from the beginning and after short time it's falling with UNKNOWN ERROR message.
My php info can be checked here
http://paulstyler.com/testphp.php