I don't actually rip extras that often, but I do have a fairly good scheme, I think.
I tag them as:
[Media Sub Type]=[Movie]
[Series] = Movie's Name
[Name] = Movie's Name
Then, I do a Get Movie & TV Info on them to fill in the metadata with, at least, that of the main movie.
After that is done, I change them again so that:
[Name] = The title of the extra, if available, or a generic description of it that I make up.
Sometimes I have this in the Filenames, or sometimes I name them manually by playing them and looking for a "Title Card" kind of thing in the opening credits.
Then, I do a Get Movie & TV Info on them again, one at a time this time, and see if the Extra is actually in the online databases. I've found a surprising number of them are, if you can get the search for the Name of it right. Shorter "making of" things don't usually show up, but sometimes they do if they have a "title".
Anyway, at least they all have metadata that matches the Movie, and some of them might have more.
Then, I apply:
[Genre]=[Extras]
You can now do this while preserving the existing Genres from the lookup, which is nice, by tagging them in the Panes and holding Control down as you check the box.
Then, I have my regular Movies View set up with the following Files To Show filter:
-[Genre]=[Extras]
And I made a special Media View just for Extras that searches for:
[Media Sub Type]=[Movie] [Genre]=[Extras] ~sort=[Series],[Episode],[Name]
It has [Series] as the Category in my Media Views for this, so you can browse them by "Movie" in Theater View and whatnot. If you're real into it, you can add Cover Art manually by searching for the movie's name on Google Images.