Federated media multipart/mixed JSON object shouldn't be required to be empty

Link to problem area:
https://github.com/matrix-org/matrix-spec/blob/2cbf6067a6d50ea2160e72d2a570f2cb8eeb0a20/data/api/server-server/content_repository.yaml#L44-L45
https://spec.matrix.org/v1.11/server-server-api/#get_matrixfederationv1mediadownloadmediaid

Issue
MSC3916 point 4 describes the federated multipart/mixed media metadata JSON object as

No properties are yet specified for the JSON object to be returned. One possible use is described by https://github.com/matrix-org/matrix-spec-proposals/pull/3911.

However the spec entry just says 'Currently, this always be an empty object.'

Expected behaviour
Either mention that there are currently no standard fields for the object in spec, or have an explanation similar to other areas in the spec where objects are allowed extending with keys that follow Java package naming convention.
afaik current server implementations already just parse the JSON object but don't enforce it to be empty.

Start with data/api/server-server/content_repository.yaml at lines 44-45 and compare the rendered media download endpoint with MSC3916 point 4. Update the specification so it no longer incorrectly requires the multipart metadata object to be empty, using wording consistent with the proposal or existing extensible-object guidance. Done when the source and rendered API description agree.