Show downloads denied with 'too many files' or 'too many megabytes' as queued and automatically requeue them every so often.
See the Changelog for list of features and fixes currently only available in the nightly builds.
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 :)