Downloads the raw bytes of one attachment from a sent message, returned with the attachment's own content type and a Content-Disposition header carrying its filename. The message must have content storage enabled and the attachment is available for up to 30 days after sending. A 404 indicates the message has no stored content or no attachment with this ID; a 425 indicates the attachment is still being stored and the request can be retried shortly; a 410 indicates the attachment was stored but has since expired.