Stop parsing the unspecced type parameter on thumbnail requests. (#15137)

Ideally we would replace this with parsing of the Accept header
or something else, but for now just make Synapse spec compliant
by ignoring the unspecced parameter.

It does not seem that this is ever sent by a client, and even if it is
there's a reasonable fallback.

changelog.d/15137.removal

@@ -0,0 +1 @@
+Remove the undocumented and unspecced `type` parameter to the `/thumbnail` endpoint.

synapse/rest/media/v1/thumbnail_resource.py

@@ -69,7 +69,8 @@ class ThumbnailResource(DirectServeJsonResource):
         width = parse_integer(request, "width", required=True)
         height = parse_integer(request, "height", required=True)
         method = parse_string(request, "method", "scale")
-        m_type = parse_string(request, "type", "image/png")
+        # TODO Parse the Accept header to get an prioritised list of thumbnail types.
+        m_type = "image/png"
 
         if server_name == self.server_name:
             if self.dynamic_thumbnails: