MultipartRelatedStreamProvider
An MultipartStreamProvider suited for reading MIME body parts following the
multipart/related media type as defined in RFC 2387 (see http://www.ietf.org/rfc/rfc2387.txt).
Gets the HttpContent instance that has been marked as the root content in the
MIME multipart related message using the start parameter. If no start parameter is
present then pick the first of the children.
public MultipartRelatedStreamProvider()