Implement body field as media caption (MSC2530)

[frebib]

Your use case

What would you like to do?

Implement matrix-org/matrix-spec-proposals#2530

Why would you like to do it?

Because it's part of the spec and Beeper users/various bridges send images in that format, which are hard to view in EW/ED without MSC2530 support

How would you like to achieve it?

Probably the same way that most other clients do- render the text directly under the image

Have you considered any alternatives?

No response

Additional context

No response

[dbkr]

I've given this 'needs product' more to decide when we do it & then hand it off to design: it's the spec now, so we probably ought to to it some time.