To support multipart-mixed content types, we need to introduce another level of content in the
requestBody definition. In the example below, we are defining a requestBody type of multipart/mixed and
inside the content definition, we have multiple content types, one for each Content-Type.
This syntax only makes any sense under the multipart/mixed content type but may be help simplify multipart/form-data
and possibly mulitpart/related and multipart/alternative, if we choose to support those.