Outlook is supposed to be able to handle winmail.dat files, as it is its own proprietary format for encapsulating emails and additional data. However, attachments will sometimes still show up as a winmail.dat file.
It’s really easy. A quick solution I have found to this if you have a somewhat recent version of Microsoft Outlook is simply forwarding the email to yourself. The attachment will magically appear in the right format again.
This is assuming you’re using default settings, where Options > Mail > Message Format > When sending messages in Rich Text format to Internet recipients is set to Convert to HTML format.
That’s it! Give it a try and let me know how it goes!