A file index tool can be add to torrent clients, letting it scan all files in a user-selected folder and tag them with torrent info. Then how can it find the torrent info for a given file? Maybe we need a central server, maybe some hacker will invent a distributed reverse torrent search network, to list what playlists has a specific song. I don't know if someone has already invented this. I think it can be the final answer to any distributed file system driven by user uploads.