There is stacktrace all over the place about refusing to msgpack deserialization  for
some reason. Our default configuration is supposed to already allow it.
The doc says it supports both the name and the mimetype .
So as a tryout/short in the dark, here we go.
It's not tested though.
ContentDisallowed('Refusing to deserialize untrusted content of type msgpack (application/x-msgpack)')
Related to T3632