That may open in G-Mail if you forward it to a G-Mail account. The reason for that file is likely from an old version (or incorrectly configured) of Outlook that is configured to send the message both in text and HTML formats. The winmail.dat file more often than not is a duplicate of what appears in the body of the message.
Generation of that file ONLY happens with Outlook as the sending client.
There is nothing you can do to avoid it short of having a heart to heart with the sender about his Outlook configuration. My guess is that it's a really old version of Outlook. As I mention in my previous post you could forward the attachment to G-Mail or open it in Outlook, but I think you will be disappointed at what you find.