2.1.3.3.8.3 Calendar Message with Inline Attachments
When the best body format type of a calendar item or meeting message is RTF and there are inline attachments, MIME writers SHOULD generate a MIME entity with "multipart/related" for the value of its Content-Type header and two or more child entities, as follows:
The first child entity is a "multipart/alternative" MIME content-type structure generated as specified in section 2.1.3.3.8.2.
Subsequent child entities are generated from the Message object's inline Attachment objects. Each entity MUST be generated as specified in section 2.1.3.4.1.
A MIME writer MAY<75> generate a MIME entity that has "multipart/mixed" for the value of its Content-Type header.