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.
We are a relatively new Front user and so far things are going really well, with the exception of the winmail.dat problem. Lots of our customers still send us orders attached as winmail.dat files and Front can't handle these. This is causing problems for us and we have little to no way of working around it.
I completely understand that winmail.dat is an awful prehistoric hangover from the days when Microsoft was in denial about the Internet and they really shouldn't have done that shit. Unfortunately telling our customers that we can't take their orders because their email system is broken and sending them an (incomprehensible to them) knowledgebase article to fix it before we can receive documents from them just doesn't work. They don't understand the problem because "everyone else can read their attachments OK" and "you used to be able to read them OK before you changed email systems".
It is horrible, but some other email clients have support for this format is a bit of a mis-statement, Gmail, Outlook, even the 14-year old Thunderbird client we used before Front can grok winmail.dat. Dealing with this brokenness is sadly a baseline requirement even in 2018.
Please just npm install node-tnef, you know you want to!