Slow load in media page of WordPress admin when having huge posts table

[Twellve_Million]

Slow load can be traced to this function.

// get_available_post_mime_types
// wp-includes/post.php

From what I understand, this function tries to get all of the available mime type of attachment post type. When posts are over a hundred thousand, this is causing media page in admin to load in about half a minute (hosting performance relative).

I'd like to suggest a transient in this function. Will open a PR if this bug is approved.