The poster_id field is not selected when grabbing the attachment data in file.php. This is used to check whether the attachment belongs to the user later on:
This results in a PHP notice and phpBB outputting the following error to the user: "The selected attachment does not exist anymore." This is what's causing http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=44811. David couldn't reproduce the issue due to the a_ permission check in the statement.