Render captions to m.file, m.image, m.video, and m.audio (#2059)

Render captions to m.file, m.image, m.video, and m.audio (#2059)

* Add rendering image captions

* Handle sending captions for images

* Fix caption rendering on m.video and m.audio too

* Remove unused renderBody() parameter

* Fix m.file rendering body instead of filename where possible

* Add caption rendering for generic files

+ Fix video and audio not properly sending captions

* Use m.text for captions & render on demand

* Allow custom HTML in sending captions

* Don't *send* captions

* mvoe content const into renderCaption()

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Co-authored-by: Ajay Bura <32841439+ajbura@users.noreply.github.com>