It's not very practical but Spotify does allow you to upload your own music: - on the desktop app, create a new Playlist - add local files to that Playlist -

It's not very practical but Spotify does allow you to upload your own music: - on the desktop app, create a new Playlist - add local files to that Playlist - while the desktop is online, open Spotify on the mobile device you plan on using, find that Playlist on the playlists section and press download Now that playlist is available offline in your mobile device too.