This is another small but stupid limitations in iPad. This is the Apple way and it is really working for Apple and nobody really worry about it or complain about it. If someone complain about it they he/she is a wired person. Anyways lets see the quick solutions to email attachment problem in iPad until Apple provide a upgrade.
If you receive a email with an attachment then you can forward the message with or without the Attachment. So you can attach a file and send a email from your MAC Computer or even Windows computer to iPad and then you will be able to add and send the email attachment from your iPad. This is a bit funny solution just like one we provided for the iPhone SMS forwarding problem we had initially.
Apple has a strong App Community, which can provide any kind of solution for the Apple limitations. GoodReader is a better solutions for email attachment problem as well as viewing Windows files and any large PDFs without any problem.