Can I dictate if an attachment is included in the text or as a thumbnail?
One way of dictating the choice is to choose what format you send the mail in. If you send mail as plain text then the attachment will need to be opened by the receiver (although the receiver's mail client may choose to provide a preview with the message, much like Mail.app does for some images and PDF documents). If you send the message as rich text (Format -> Make Rich Text) you can choose to place images etc. in the message and they will be visible in place to the receiver (depending on the capabilities of the receiver's client).