Improve performance of get_available_post_mime_types

[srtfisher]

Description

I would like to see the performance of get_available_post_mime_types() improved as the underlying database query can be quite expensive on large sites:

SELECT DISTINCT post_mime_type FROM wp_posts WHERE post_type = 'attachment' AND post_mime_type != ''

This is being flagged by New Relic, too.

Use Case

When a user views the media library and the underlying call to get_available_post_mime_types() is made the function should return quickly from the cache.