Switch to mime-db as source of extension => MIME map

Switch to mime-db as source of extension => MIME map

The Apache MIME type map is not actively maintained anymore, so
this switches to jshttp/mime-db, which seems to be the de-facto
standard in this area now. This avoid the need to patch in our
own MIME types over time.

The preference algorithm is based on:
https://github.com/jshttp/mime-types/blob/47b62ac45e9b176a2af35532d0eea4968bb9eb6d/index.js#L154

Closes GH-5764.