Use IMail.Visuals collection to retrieve 'inline' images and IMail.NonVisuals to get real 'attachments' (content-disposition set to attachment).
Mail.dll uses 4 collections to group email attachments:
IMail.Attachments – all attachments found in the email (this list basically combines all other ones).
IMail.Visuals – visual elements, attachments that should be displayed to the user. This list contains images used by HTML content for example.
IMail.NonVisuals – non visual elements, those are real attachments, they have content-disposition explicitly set to attachment by the sender.
IMail.Alternatives – alternative content representations, e.g. ical appointment.