Mail App fix confirmed
I can confirm the attachment bug in the HTC Mail app seems to have been fixed in the 4.22.651.1 update.
When setting up an Exchange Activesync account in the HTC Mail app (at least while using Gmail set as an Exchange Activesync account). I can download and view the attachment and there is no "Attachment is Corrupt" error. I am also able to foward an email with an attachment from my Inbox. However if I go into the Sent Mail folder and try to forward a previously sent email it does not send and I get the "Unable to Send Email" error.
The HTC Mail app is now at version 2.00.0037.60865. I'm not sure what the version was before the update though.