We have the same problem with one user in our office. Her QB invoices go to customers as a win.dat file.

Mine go out as a regular pdf file. Our QB and Outlook settings are identical. If she sends a file to a customer who receives it as win.dat, and then I send it, they get a pdf from me. This would clearly indicate that it is a Quickbooks problem and not an Outlook problem as we are being told.

We shouldn't have to do 'work arounds' for a program we pay in excess of 2.5K a year for. And it's a problem that is persisting from 2021 into the 2022 version. Intuit, when will this get fixed?