Windows (debug): SoulseekQt-2015-3-25-debug.7z
And musepack too .
This complements the other feature request to specify file type/extension.
However, scanning of files, namely lossless ones, can be slow, depending on the format's architecture for metadata support, so either add a throttling mechanism or allow the user to disable it entirely. In fact, you should allow disabling it even for mp3.
I think that there were some efforts to create multi-format libraries capable of scanning just about anything, but I don't know any name of the top of my head. Your best best lies probably on SoX or ffmpeg projects. Additionally, you can try each format's own encoder/decoder/libray - there are only about a dozen of popular and moderately popular music formats :)