I suspect the error is because the fact that user id is not available and the die statement is causing the upload error.
I would try to look into what is available in $_POST, although I don’t know exactly how you can do this because all of this is happening in an AJAX request (this is the reason your die statement is causing an error)
You could try outputting the data to the error log
ob_start(); print_r($_POST); error_log(ob_get_clean());
You might find the user ID included in $_POST that is submitted for the AJAX request.
