Use proper MIME type for gz/bz2 downloads (BB-689)

The MIME types for gz/bz2 downloads seems to be application/x-tar. Not only is this incorrect, it messes up Safari. If the file name does not end in .tar, it adds it to be "helpful" (hey, you downloaded a tar file, right?).

For example, downloading foo on tag 1.0 as a bz2, results in a file called "foo-1.0.bz2.tar". You should use application/x-bzip2 for bz2 and application/x-gzip for gzip.