Do not 500 if the content-length is not provided when uploading media. (#8862)

Instead return the proper 400 error.

changelog.d/8862.bugfix

+Fix a longstanding bug where a 500 error would be returned if the `Content-Length` header was not provided to the upload media resource.

synapse/rest/media/v1/upload_resource.py

@@ -44,7 +44,7 @@ class UploadResource(DirectServeJsonResource):
         requester = await self.auth.get_user_by_req(request)
         # TODO: The checks here are a bit late. The content will have
         # already been uploaded to a tmp file at this point
-        content_length = request.getHeader(b"Content-Length").decode("ascii")
+        content_length = request.getHeader("Content-Length")
         if content_length is None:
             raise SynapseError(msg="Request must specify a Content-Length", code=400)
         if int(content_length) > self.max_upload_size: