Perhaps I misunderstood (please correct me if that is the case), but
MailMessage.Resources are two different collections, so an attachment will never be present in both (and the same applies to a linked resource). Additionally, the placement in the MIME message tree determines whether a MIME entity is an attachment or a resource.
But it looks like you might find our low-level
MimeEntity API useful. Unlike
MailMessage, this gives you access to the original MIME message structure - see the MimeExplorer sample to get started. However, this API doesn't support TNEF messages - in order to work with these, you would have to convert them to MIME via