Fix import of raw images
- media-manager: check for null before dereferencing
- don't consider application/x-raw-disk-image as an installer media
- check content type using ContentType.is_a so that more specific content types are matched correctly
- check for installer media first since a cd-image "is a" raw-disk-image (and octet-stream caches everything)
- only consider supported compression types (which means only gzip for now)
Fixes #860 (closed)