I think it can be a good idea to have ad admin interface for defining custom mime types, stored in database (best) or other file than "filelib.php".
It would be useful for beginners who want to define new mime types but doesn't want to edit php files; and however it make easier the upgrade process, because now I have to re-edit the filelib.php at every version upgrade. I now there is the possibility to upgrade via CVS, but this is not so easier (and well known) for everybody.