2.1.3.3.7 HTML Text Message Body from RTF with Inline (OLE) Attachments
When the best body format type is RTF and inline (OLE) Attachment objects exist, MIME writers SHOULD generate a MIME entity with multipart/related for the value of its Content-Type header and three or more child entities, as follows:
The first child entity is a structure with a MIME content-type of "multipart/alternative", as specified in section 2.1.3.3.5.
Subsequent child entities are generated from the Message object's inline Attachment objects. Each entity is generated as specified in 2.1.3.4.4, because inline Attachment objects in RTF messages are OLE attachments.