Winmail.dat is a compression format used by old versions of Microsoft Outlook or MS Exchange Server. It's now deprecated and recent Microsoft products no longer support it.
Winmail.dat is a proprietary attachment format created by a Microsoft Outlook client. Gmail and Outlook clients have the ability to read these documents, but few other email clients do, due to its proprietary nature. This is a pretty universal frustration external to Front that can only be remedied on the Outlook user's end.
Although some email clients were able to build support for this format, Front does not currently support it. Front lacks support for Winmail.dat, and as many other major email clients also lack support (Mac Mail: https://support.apple.com/en-us/HT201773)
Microsoft's support page describes a way for senders to disable this format, if you are able to ask them to re-send their email.