Why does a word doc attachment on my Wife’s phone show as winmail.dat?

Note: I understand the general issues about Outlook encoding attachments, but this is about a specific device. Our school sends out a newsletter as a .doc attachment. My Wife and I both have iPhone SE’s. I can open the attachments fine, but on my Wife’s it shows as a winmail.dat attachment and she can’t open it. We both have the same mail provider (a private mail server). Even if I forward the email I received, her phone shows it as winmail.dat. It opens OK if she views it on webmail so this is very specific to just her phone. Her iPhone is up to date, mine is running the iOS 12 beta, but she has been having this problem before I installed the beta so I don’t think it’s related to the iOS version. As far as I can tell, there are no settings on the phone that might control this. Any ideas?