18.104.22.168.18 Payload Properties
To set the value of the PidTagAttachPayloadClass property ([MS-OXCMSG] section 22.214.171.124) or the PidTagAttachPayloadProviderGuidString property ([MS-OXCMSG] section 126.96.36.199), clients SHOULD<124> write an X-Payload-Class and an X-Payload-Provider-GUIDheader (2), respectively. Such headers (2) SHOULD<125> be written to a MIME entity that will be analyzed as an attachment, as specified in section 188.8.131.52.
MIME readers SHOULD<126><127> copy these header values to the values of the corresponding properties. MIME readers SHOULD<128> copy these headers (2) when they appear on a MIME entity that is analyzed as a message or message body, rather than as an attachment.