I've read from many places that if Spotify doesn't have songs in its database that I want, as a Premium user I can upload my own files from my computer to "fill in the gaps." I tried doing this, but after I upload the songs onto my computer, I can only listen to them from my computer.

I for some reason was not able to click on the "+" sign to add these songs to My Library. Instead, I put these local files into playlists so that I could access them on my mobile device through the playlist, but when I try pressing the song title from my Galaxy S3, it says "This track is unavailable."

I really want to upload a handful of mp3 files to put on my playlists, but I don't know why I am having this problem of not being able to access my local files (from my computer) on my phone.