MSC2530: Body field as media caption

proposals/2530-body-as-caption.md

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+# Body field as media caption
+
+This is an alternative to [MSC2529](https://github.com/matrix-org/matrix-doc/pull/2529).
+
+## Proposal
+
+This proposal defines a new field `filename` and allows the `format` and
+`formatted_body` fields (from `m.text`) for all media msgtypes (`m.image`,
+`m.audio`, `m.video`, `m.file`).
+
+If the `filename` field is present in a media message, clients should treat
+`body` as a caption instead of a file name. The `formatted_body` field should
+also be supported and work the same way as they do in `m.text` messages.
+
+## Comparison to MSC2529
+
+While this approach isn't backwards-compatible like MSC2529, it's simpler to
+implement as it doesn't require handling relations.
+
+Implementing relation-based captions would be especially difficult for bridges.
+It would require either waiting a predefined amount of time for the caption to
+come through, or editing the message on the target platform (if edits are
+supported).
+
+Additionally, some clients already render the body (as a file name), so the
+new-style caption would already be visible in those cases.
+
+The format proposed by MSC2529 would also make it technically possible to use
+other message types without changing the format of the events, which is not
+possible with this proposal.
+
+## Potential issues
+
+The spec for `m.file` already defines a `filename` field, but the spec also
+says `body` should contain the file name. This means old `m.file` messages
+could be misinterpreted to contain a caption, even though it's just the file
+name. To avoid this problem, we could require that `body` and `filename` must
+be different for `body` to be interpreted as a caption.
+
+Like MSC2529, this would be obsoleted by [extensible events](https://github.com/matrix-org/matrix-doc/pull/1767).
+In my opinion, unless extensible events are planned to be pushed through in the
+near future, we shouldn't block basic features like captions on extensible
+events hopefully eventually happening.