Adding inline image into an email - Transport Agent

I am trying to add an inline image into an email and then get a content id of that image so I can use it as the source of an html image.

The approach i am taking is creating an attachment (which will create a new mime part) but I am unable to give that mime part a content-id so i can reference it back in the email.

Am I taking the correct approach. If so how can i add a mime property (content-id)? Also I would like to set the attachment type to "Inline" so it does not appear as an attachment but the compiler says the Attachment.AttachmentType is readonly 

Thanks in advance.