On attachment pages for audio and video, display players. Currently, only a link is displayed. Add minimal CSS rules for compatibility with 2011, 2012, and 2013 themes.

In `prepend_attachment`, add logic to support attachment types that are not `image`.

In `get_post_class()`, don't add the `has-post-thumbnail` class for attachment pages.

Fixes #27243.



git-svn-id: https://develop.svn.wordpress.org/trunk@27622 602fd350-edb4-49c9-b593-d223f7449a82