If you're comfortable working through the code a bit, I'd modify file-storage for this purpose.
If you're willing to add categories to file storage, that would probably do the job for you. This might also be a useful addition for other people, so you can see if others are interested in this functionality. If they are, then you may be able to get it included in the default file-storage.
I believe on another thread you figured out how to get searching working on file-storage.
