I can't play local files which are added to Spotify. They show up under "Local files", and I can add them to playlists, but if I try to play them, I get the error message "This song is not available. If you have the file on your computer you can import it." - which is exacly what I have done.
The files are standard mp3 files, I have re-encoded them just to see if that helped, it did not. 320 and 192 kbs files, 44.1 kHz, stereo/jntstereo. They will play in any other application.