I have an S35S and I've been playing around with rioutil on my linux machine at work and Rio Music Manager / Rio Taxi on my Windows machine at home.
I noticed that non-mp3 files uploaded with rioutil will show up in the Rio Music Manager as a music, and they do not show up at all in Rio Taxi.
So I decided to play around a little bit. I checked out the rio_file_t struct for a file uploaded to my S35S with the Rio Taxi utility and it had a different bits and type field than a file uploaded with rioutil.
I modified downloadable.c to set the bits and the type to the same values that Rio Taxi sets them to, recompiled rioutil, and now non-mp3 files that I upload show up in the Rio Taxi utility and not the music manager utility.
I'm submitting a patch with my modified downloadable.c, but since I only have a S35S to test with I'm not so sure it would work with the other players.
Thanks for all your great work on this library.