This simplifies the code since we don't have to keep large mappings of extensions and MIME types.
We still keep the ability to override the mappings for:
- filling in entries not present in the package, for e.g. ".azw3"
- picking preferred extensions, for e.g. MimeTypes provides ".conf" as a possible extionsion for "text/plain", and while that is correct, ".txt" would be preferrable
- compatibility reasons
