[rin]

Great news. You just helped me to get the script working. Here's the code:

$limit = 10000000;
$query = $vbulletin->db->query_first("
	SELECT current_bandwidth FROM " . TABLE_PREFIX . "user 
	WHERE userid = " . $vbulletin->userinfo[userid]);

$add = $query['current_bandwidth'] + $attachmentinfo['filesize'];

$vbulletin->db->query_write("
	UPDATE " . TABLE_PREFIX . "user 
	SET current_bandwidth = $add
	WHERE userid = " . $vbulletin->userinfo[userid]
	);
if($add > $limit)
{ exit(); }

Working! One more thing I was wondering, if exiting the script like this, it will cache a blank attachment. So in case I up the limit a bit, it will not reload the actual attachment, but still show the blank one. I guess there's no easy way around this except for emptying the cache.
Also, currently, when the limit is crossed, it will show a blank attachment. Is it possible to use the space for a proper message telling the user that their bandwidth has exceeded the allowance?
Simply adding a print wouldn't do the trick.