Modify attachment's poster_id for get_submitted_attachment_data

Currently, $post_data['poster_id'] is used in

$message_parser->get_submitted_attachment_data($post_data['poster_id']);

in posting.php, line 614.

Because of the nature of my anonymous posts extension, the poster_id is still 1 at this stage.  When users edit posts, they receive a "You are not allowed to access this file." backtrace error.  Allowing this var to be modified by an event would solve this problem for me, and anyone else who wants guests to be able to use attachments.