Very helpful. I've added music (local files) to my desktop Spotify. I can see them on my mobile device, however some of them are greyed out and will not play on mobile device (yes, they play on desktop). I saw some mention of the mp4 format not working, but the ones I'm having trouble with are in mp3 format. Any thoughts? It's a handful of songs that I'd really love to listen to on my mobile.
YES, in a way. Assuming your music is on a PC that you have access to and any devices you want to share the music with can get on the same wifi connection as the PC.
You will need to use the desktop app and go to settings and ADD SOURCES from which spotify can locate your LOCAL MUSIC.
Make a Playlist of these local files. Go to that playlist on any devices that you have Spotify connected to (You can have 3 offline devices) Go to the playlist you created and make sure you check the toggle at the top of the playlist that says DOWNLOAD or (formerly) AVAILABLE OFFLINE
This should do it You will need suffiecient storage capacity and not be some noob with an iPhone.