That's still possible, although not as seamless as it was on Google Play music where you could upload music to Google's servers and stream just like their offic

That's still possible, although not as seamless as it was on Google Play music where you could upload music to Google's servers and stream just like their official library. In Spotify, you have to point the desktop client to a local folder with your music. I have then made a special playlist where I put those songs and mark that as available offline on my phone. I'm pretty sure it only syncs while on the same LAN. I have some of that local music in other playlists as well, and it mostly works fine. I don't use it a lot, but I think the key is to make sure it's available offline on the device you want to play it on.