Switch to mini_mime gem from mime-types

[Fryguy]

mini_mime is a much more performant replacement for the mime-types gem. It uses the same database as mime-types, but loads it in a much more efficient manner, improving the number of allocations, memory, and speed. The mail gem, which is a core component of Rails, has already switched to it. See mikel/mail#1059 for more details on the mail gem changes, as well as the some performance numbers. Bonus is that mini_mime supports Ruby versions all the way back to 1.8.7, so there's no need for a caveat anymore.

cc @SamSaffron

[Fryguy]

Performance details (quoting @SamSaffron from the referenced PR)

The mime types gem (even with the magical columnar store by @jeremyevans) is a memory hog.

In particular on boot, mime/types/columnar will allocate 109k objects and end up with a retained count of 31K objects.

This bloat leaves objects that need to be marked and swept every major GC and bloats ruby processes.

To circumvent mini_mime was created.

It uses the exact same database as the full fledged mime types and is capable of only loading stuff on demand with a practical, safe and bound in-memory cache.

mini_mime will allocate 398 objects on boot and only retain 62 (due to rubygems inefficiency)

In table form:

	boot allocations	boot retained	lookup	lookup uncached
mini_mime	398	62	641K/s	33K/s
mime-types	109796	31165	361K/s	361K/s

The performance of mini_mime is pretty good, cached lookups are faster than the mime types gem and uncached lookups are 10x slower. (which is really not a huge issue considering the memory savings)

[twalpole]

LGTM, and no need for fixing the broken sorting from mime-types either apparently - thanks!