Mark,
You're correct in assuming that in ev6.0 that if you want to create a sep retention for a "type" of file, that you need to create a sep folder and apply evpm perms to that. In ev7.0 you can use the orchesetria modules to filter content and auto assign a retention cat to the item. If you've got ev6.0 and DA, the only way to figure out what's in an image or wav file is to actually listen to the darn thing. Sucks, I know.
micah