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:

	if ($attachment['is_orphan'])

	{

		// We allow admins having attachment permissions to see orphan attachments...

		$own_attachment = ($auth->acl_get('a_attach') || $attachment['poster_id'] == $user->data['user_id']) ? true : false;

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.