Storage : fix: download audio/wav instead of source mimetype (#25)

We support a bunch of different upload mimetypes, such as .m4a. This is great for uploads but for downloads we should use a more widely supported audio format.

Rock just had his vibe killed trying to drag an .m4a into Ableton - this fixes that!

https://github.com/stemstr/Storage/commit/cfb075ccb898cd0a32a226c5b5cf5b4152aa2175