Body data missing in decodedMessage call

I have a multipart message and in the MEMessageSecurityHandler.decodedMessage(forMessageData:) function call, I can access all of the headers of the email, but when I get to the body section, the actual content of the email is missing.

I've dumped the data out to a file and can see that there is an X-Apple-Content-Length header been added, but the content of the multipart email

--=-=W2HxPVXDSaWZZw=-=
X-Apple-Content-Length: 11
Content-Type: application/pgp-encrypted


--=-=W2HxPVXDSaWZZw=-=
Content-Transfer-Encoding: 7Bit
Content-Disposition: inline;
	filename=openpgp-encrypted-message.asc
Content-Type: application/octet-stream;
	name="openpgp-encrypted-message.asc"
X-Apple-Content-Length: 9608


--=-=W2HxPVXDSaWZZw=-=--

How can I access the content of these parts to decrypt the message?