Represents a multipart/related (RFC2387) incoming mime message. A
multipart/related message is used to hold a root or start part and other
parts which are referenced from the root part. All parts have a unique id.
The type and the id of the start part is presented in parameters in the
message content-type header.
The type parameter as it was read from the content-type header of the
multipart/related message. A well formed multipart/related message always
has this parameter. This is the type of the root part of the message. If
a content-type header is presented in the root part as well it should
hold the same value.
the type parameter of the content-type header of the message,
null if there was no such parameter