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.
 

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