Spotify certainly allows self-uploaded tracks (I know, I use them daily), though it's certainly not obvious how you do so and may require a paid subscription.
Really? I've never seen that feature--I know you can have your own local tracks stored on device, but as far as I can tell, those don't carry over if, for example, you're using Spotify Connect.
I didnt know this could be done either - heres a HTG link on how to do it.