MSC2530: Body field as media caption (#2530)

* Proposal to use body field as media caption

* Add paragraph about relation-based captions being difficult for bridges

* Clarify how to treat body when filename is not present

* Refactor proposal text

* Fix heading size

* Add problem statement

* Add links to and quotes from current spec

* Adjust wording and quote m.audio body spec

* Clarify that m.location and m.sticker are out of scope for this proposal

* Add examples and summary of changes

* Fix JSON syntax in example

proposals/2530-body-as-caption.md

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+# Body field as media caption
+
+When sending images or other attachments, users often want to include text to
+convey additional information. Most chat platforms offer media captions as a
+first-class feature, allowing users to choose the attachment and write text,
+then send both together in one message.
+
+Matrix currently does not enable this on the protocol level: at best, clients
+can emulate the behavior by sending two messages quickly; at worst, the user
+has to do that manually. Sending separate messages means it's possible for
+the second message to be delayed or lost if something goes wrong.
+
+## Proposal
+
+This proposal allows the `filename` field from [`m.file`], and the `format` and
+`formatted_body` fields from [`m.text`] for all media msgtypes (`m.image`,
+`m.audio`, `m.video`, `m.file`). This proposal does not affect the `m.location`
+msgtype, nor the separate `m.sticker` event type: stickers already use `body`
+as a description, and locations don't have file names.
+
+If the `filename` field is present in a media message, clients should treat
+`body` as a caption instead of a file name. If the `format`/`formatted_body`
+fields are present in addition to `filename` and `body`, then they should take
+priority as the caption text. Formatted text in media captions is rendered the
+same way as formatted text in `m.text` messages.
+
+The current spec is somewhat ambiguous as to how `body` should be handled and
+the definition varies across different message types. The current spec for
+[`m.image`] describes `body` as
+
+> A textual representation of the image. This could be the alt text of the
+> image, the filename of the image, or some kind of content description for
+> accessibility e.g. ‘image attachment’.
+
+while [`m.audio`] describes it as
+
+> A description of the audio e.g. ‘Bee Gees - Stayin’ Alive’, or some kind of
+> content description for accessibility e.g. ‘audio attachment’.
+
+In practice, clients (or at least Element) use it as the file name. As a part
+of adding captions, the `body` field for all media message types is explicitly
+defined to be used as the file name when the `filename` field is not present.
+
+For `m.file` messages, the [current (v1.9) spec][`m.file`] confusingly defines
+`filename` as "The original filename of the uploaded file" and simultaneously
+recommends that `body` is "the filename of the original upload", effectively
+saying both fields should have the file name. In order to avoid (old) messages
+with both fields being misinterpreted as having captions, the `body` field
+should not be used as a caption when it's equal to `filename`.
+
+[`m.file`]: https://spec.matrix.org/v1.9/client-server-api/#mfile
+[`m.text`]: https://spec.matrix.org/v1.9/client-server-api/#mtext
+[`m.image`]: https://spec.matrix.org/v1.9/client-server-api/#mimage
+[`m.audio`]: https://spec.matrix.org/v1.9/client-server-api/#maudio
+
+### Examples
+<details>
+<summary>Image with caption</summary>
+
+```json
+{
+    "msgtype": "m.image",
+    "url": "mxc://maunium.net/HaIrXlnKfEEHvMNKzuExiYlv",
+    "filename": "cat.jpeg",
+    "body": "this is a cat picture :3",
+    "info": {
+        "w": 479,
+        "h": 640,
+        "mimetype": "image/jpeg",
+        "size": 27253
+    },
+    "m.mentions": {}
+}
+```
+
+</details>
+<details>
+<summary>File with formatted caption</summary>
+
+```json
+{
+    "msgtype": "m.file",
+    "url": "mxc://maunium.net/TizWsLhHfDCETKRXdDwHoAGn",
+    "filename": "hello.txt",
+    "body": "this caption is longer than the file itself 🤔",
+    "format": "org.matrix.custom.html",
+    "formatted_body": "this <strong>caption</strong> is longer than the file itself 🤔",
+    "info": {
+        "mimetype": "text/plain",
+        "size": 14
+    },
+    "m.mentions": {}
+}
+```
+
+</details>
+
+### Summary
+* `filename` is defined for all media msgtypes.
+* `body` is defined to be a caption when `filename` is present and not equal to `body`.
+  * `format` and `formatted_body` are allowed as well for formatted captions.
+* `body` is defined to be the file name when `filename` is not present.
+
+## Potential issues
+
+In clients that don't show the file name anywhere, the caption would not be
+visible at all. However, extensible events would run into the same issue.
+Clients having captions implemented beforehand may even help eventually
+implementing extensible events.
+
+Old clients may default to using the caption as the file name when the user
+wants to download a file, which will be somewhat weird UX.
+
+## Alternatives
+
+### [MSC2529](https://github.com/matrix-org/matrix-spec-proposals/pull/2529)
+
+MSC2529 would allow existing clients to render captions without any changes,
+but the use of relations makes implementation more difficult, especially 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).
+
+The format proposed by MSC2529 would also make it technically possible to use
+other message types as captions without changing the format of the events,
+which is not possible with this proposal.
+
+### Extensible events
+
+Like MSC2529, this would be obsoleted by [extensible events](https://github.com/matrix-org/matrix-spec-proposals/pull/3552).
+However, fully switching to extensible events requires significantly more
+implementation work, and it may take years for the necessary time to be
+allocated for that.
+
+## Security considerations
+
+This proposal doesn't involve any security-sensitive components.
+
+## Unstable prefix
+
+The fields being added already exist in other msgtypes, so unstable prefixes
+don't seem necessary. Additionally, using `body` as a caption could already be
+considered spec-compliant due to the ambiguous definition of the field, and
+only adding unstable prefixes for the other fields would be silly.