Yes, supporting that Microsoft abomination called "winfile.dat" is on my to-do. Thanks.
I've been plagued by this problem recently when emailing attachments to myself from my desktop Outlook 2003.
The general winmail.dat problem that people encounter is usually solved by forcing the email to be sent from Outlook as either Plain Text or HTML, and not "Rich text/RTF". In my case, I have tried sending JPGs and a ZIP file of those JPGs to my Aqua mail and they only come through as winmail.dat files when I send them in an HTML email.
However, I do get the proper attachments when I send the email from Outlook as a Plain Text email. There seems to be something wrong with the way Aqua Mail interprets the HTML version of the emails sent from Outlook 2003. (Yeah, it's an Outlook issue too, but we have to adapt...
Hopefully this can be fixed soon.
UPDATE: I sent the files normally as attachments in a (default) HTML email to my GMAIL account, and when I received the email via Aquamail the attachments were fine. Hope that helps to narrow it down...