The FileTypes preferences panel simply reads attributes out of the hierarchy of files stored in /boot/home/config/settings/beos_mime/ and its subdirectories. In certain rare instances, things can go whack and your filetype associations could start acting screwy. It may become useful to have this database rebuilt. For example, if you restore your home directory from a backup and overwrite your current MIME database with a later version, you might lose some of your application-document associations, icons, etc. (it’s sort of like reinstalling Windows and then replacing the new Registry with your old Registry that you saved before reformatting, though this analogy breaks if you press on it too hard).
To try rebuilding your beos_mime database, quit any non-essential applications, leaving only the Tracker/Deskbar and a Terminal. Move your ~/config/settings/beos_mime folder elsewhere and rename/archive/etc it. You’ll also want to delete or rename ~/config/settings/MIMEPrefs_settings. Now, type:

mimeset -F -all
sync
shutdown -r -q

in a terminal. Then start praying to your favorite deity. When your computer starts back up, you should (emphasis on should) have a fresh new beos_mime database. If not, well, you’re no worse off than before. ^_^ If it did work, sacrifice your old beos_mime folder to your deity by putting it in the trash and emptying it. Always keep your computer deity happy.