There is no such thing as a CID attachment.
"cid:" protocol is used by HTML body of the message to reference images/resources attached to the message.
It specifies Content-ID of the image/resource that should be used.
For example this HTML:
<img src="cid:logo@example.com" />
...references an image, attached to the message. The image is stored as a part of the MIME tree, in most cases related to HTML body part.
It has a Content-Disposition header set to inline and Content-Id header set to logo@example.com.
You can use IMail.Attachments[string contentId] to find attachment by Content-ID:
IMail email = ...;
MimeData attachment = email.Attachments["logo@example.com"];