Ah, I see. Yeah, custom filters show up in the list under custom filters, not in the main drop down. It allows you to filter your collection by anything, but not dynamically like that.
I see what you're getting at, once we properly support multiple types of media (games, movies, music, etc.), we'll need multiple levels of filtering like that, for sure. It does get pretty complicated at that point, so I want to make sure however we implement it is easy to understand.