In `wp_generate_attachment_metadata()`, when an audio or video files contains upload-able image bits in its ID3 tags, only upload it if the image has not already been uploaded. Determine this by checking for a `_cover_hash` value in post meta that matches the `md5` representation of the bits.

This prevents uploading an album of 10 songs and subsequently uploading 10 copies of the same album cover.

Props GregLone for the new filter/filter docs: `'attachment_thumbnail_args'`.
Fixes #27573.
 


git-svn-id: https://develop.svn.wordpress.org/trunk@27863 602fd350-edb4-49c9-b593-d223f7449a82