You can create any MIME document with Mail.dll, you can use RTF content if you want (MailBuilder.Rtf property).
However I doubt that it will be handled as you want by every email client there is. What happens if you send such email from Outlook to Gmail? Does it look good in Gmail?
The global consensus seem to be to use attachments as all would expect: on the attachment list, not inside email body.
To be honest, I don't know how email clients would react on cid in "a href=" attribute. I would expect many of them block such HTML, as users could be tricked in doing something they don't want. But as, I said, I simply don't know.