BUG Attempt to access File::$allowed_extensions when uploading invalid file
BUG Attempt to access File::$allowed_extensions when uploading invalid file

Use Config to retrieve the variable instead of trying to access it
directly, and failing.

mandrew/silverstripe-forum,mandrew/silverstripe-forum,camfindlay/silverstripe-forum,camfindlay/silverstripe-forum,gordonbanderson/silverstripe-forum,gordonbanderson/silverstripe-forum,Olliepop/silverstripe-forum,Olliepop/silverstripe-forum