126.96.36.199.1.1 Recognizing a Clear-Signed Message in Internet Format
The media type of the message MIME entity is the value of the last Content-Typeheader field, as specified in [RFC2045] section 5. If the media type of the message MIME entity is "multipart/signed", the message SHOULD be treated as a clear-signed message. Additional verification steps can be performed at this point. For example, a client or server could choose to verify that the multipart/signed MIME entity contains exactly two MIME body parts, as specified in [RFC1847].