Ask the person sending you the emails with the winmail.dat attachment to stop sending you emails in rich text format and send them in plain text format instead.
winmail.dat is created by outlook when mails are composed in rich text format, but outlook is the only mail client in the world that uses that file, so if an outlook user sends a rich text mail to a non-outlook user you end up with that file that you can't do anything with.
It's nothing you can do your end (unless you start using outlook)