Images with captions

[leonerd]

It would be useful to be able to put a "caption" field with an image, to somehow be able to atomically post an image with some explaining text with it.

This feature would be especially useful for bridges and third-party protocol integrations, as for example both Slack and Telegram include this feature, so it would be good if we can bidirectionally represent it into and out of Matrix.

[leonerd]

E.g. currently the Slack bridge just posts an incoming commented image from slack by separate m.image and m.text messages:
matrix-org/matrix-appservice-slack@158a8e8

[richvdh]

matrix-purple has a similar kludge: https://github.com/matrix-org/purple-matrix/blob/master/matrix-room.c#L1193

[richvdh]

messages can now include inline images, so maybe we can use that

[auscompgeek]

See MSCs matrix-org/matrix-spec-proposals#2529 and matrix-org/matrix-spec-proposals#2530.

[ara4n]

msc2530 merged