Link to problem area: https://spec.matrix.org/v1.5/client-server-api/#get_matrixmediav3thumbnailservernamemediaid

Issue
Synapse supports a type parameter (which defaults to image/png if not provided), this is not in the spec, but has existed since this endpoint has.

matrix-media-repo does not seem to support this.

Dendrite does not seem to support this either, although declares it uses a similar algorithm to Synapse to find thumbnails.

Expected behaviour
Add the type parameter, as documented above.

As an aside, @Half-Shot rightly pointed out that this is what Accept headers are for?