Re-instantiate the `MediaElementPlayer` instance when switching mime types in playlists.

Fixes #27608.


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Forward pingback IP during pingback verification.

props tellyworth, nacin.
fixes #27613.

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Clarify inline documentation for the 'html5' case and PHPDoc return in `add_theme_support()`.

Props jond3r.
Fixes #27278.

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Clarify PHPDoc return description for `media_handle_upload()`.

Props jdgrimes.
Fixes #27519.

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