Allow fetching unfiltered allowed mime types list

[ryan]

In #11668, do_enclose() gained mime type checking. It uses get_allowed_mime_types(). However, multisite installs greatly restrict the mime list when they run check_upload_mimes() on the upload_mimes filter. do_enclose() needs access to the full list. Introducing get_mime_types() and making get_allowed_mime_types() a wrapper could accommodate this.

[ryan]

While in here we can also address #21299.

[ryan]

In [21541]:

Introduce wp_get_mime_types() for fetching the complete list of mime types. Remove the static caching of the types so that filters other than the first filter work. Use wp_get_mime_types() in do_enclose(). fixes #21299 #21594

[SergeyBiryukov]

​[UT976]