In TBird I can create a new email and add an attachment and send it everything works as it should. If I receive an email that has an attachment and then try to forward it the recipient gets an email that is full of deconstructed code. And the attachment is renamed to the name of the original email. This is not how other clients work. However if instead of using the forward button you click the drop down menu to the right of the forward button there is a redirect button. This opens the email as it does for forward but when you hit send, it sends the email with the attachment as you originally received it. So in short don't forward an email that has an attachment, redirect it.