As someone else has stated, you can add music you own (or at least, music to which you have files for) directly in to Spotify, though I'll say the reliability is "ok" if you are listening on multiple devices. My guess is they don't host those files the same as the others. Maybe if you have a server or computer that's always on, you can put the file on that, and add it to Spotify on that device, maybe Spotify will just feed you the file from your computer