There are two issues… one is whether or not it will serve something.
The answer is yet. mt-daapd will server jpg files, or pdf files, or .doc file. It just can’t get metainfo for them like artist, album, song title, etc.
In fact, the *only* files types it can currently get metainfo for is mp3, m4a/m4p, ogg and flac, and ogg and flac only if they have been compiled with the appropriate libraries.
That’s one part. So you could put “.pdf” in your extensions, and mt-daapd will add any pdf files it finds to the music library. When you connect with a client, it will see the pdf files, but there will be no metainfo — the song length will show up as continuous, and the “song name” field will be the filename of the file.
If they don’t show up at all, then the problem is that it isn’t in “extensions”, or it doesn’t have the permissions to get to it.
That’s all it can be.
Again, run it with a -d5 and watch your log.