Attachments disappearing -- becoming "winmail.dat"

We’ve been using Virtualmin for a few months now and have recently noticed that old attachments to emails are gone. The emails still exist on the server, but all attachments are shown as a .dat file named “winmail.dat”.

Everyone’s email is set to unlimited quota.

Are those attachments lost to the void? If so, is this a standard feature of Virtualmin – the deletion of old attachments even on accounts with unlimited quota?