By default wordpress only includes audio and video mimetypes when it reads your post to decide if there are any urls it should enclose. This means that if you want to enclose bittorrent in wordpress, it won’t do it without a little hacking.
in do_enclose, around line 775, there’s the line:
$allowed_types = array( ‘video’, ‘audio’ );
$allowed_types = array( ‘video’, ‘audio’ , ‘application’ );
Note that any link to some application/* will become enclosed, so this is an awful, awful hack.