Wouldn't that same logic apply to messages or identities? I was thinking it's there not for Stampie to implement multiple times, but for users of the library who didn't want to/couldn't extend the Attachment class for some reason. I'm trying to follow what [seems to](https://github.com/henrikbjorn/Stampie/blob/master/lib/Stampie/MessageInterface.php) [be the](https://github.com/henrikbjorn/Stampie/blob/master/lib/Stampie/MailerInterface.php) [convention](https://github.com/henrikbjorn/Stampie/blob/master/lib/Stampie/IdentityInterface.php), but if you think it should be removed, I'm happy to do it.