Mike is a big music fan and he uses Spotify to manage it. But it's not very good for that. Leo says that Spotify Playlists is the chief way to do it. But you're "renting" the music with a subscription, and that means a third-party app that can do it would be a challenge. Spotify is probably the best out there though, and that's not really saying much. But it's not really their business model either. It's more like a jukebox.
There are plenty of apps for curating your personal mp3s out there, but that's not what Spotify is about.