The problem is that the Default file Name setting is using the $_FILES collection, not the hidden form element:
'DefaultFileName' => "".((isset($_FILES["ItemThumb2"]))?$_FILES["ItemThumb2"]["name"]:"") ."",
assuming the hidden form element where named "hiddenItemThumb2" the above code should look like:
'DefaultFileName' => "".((isset($_POST["hiddenItemThumb2"]))?$_POST["hiddenItemThumb2"]:"") ."",