Does the job

This app does an excellent job of pulling out and making accessible files locked into winmail.dat attachments. It opens immediately when you click on a winmail.dat attachment. Once open, it works pretty intuitively, opening up Excel, PDF, or Word documents directly in those applications just by double-clicking the file. One can also save a file directly into a folder in Finder with the Save button. My one complaint is that you must do this one file at a time. It would be much better if one could select multiple files and save them with one click, nor can you click-&-drag the files; they must be Saved.

MShim about Winmail Viewer - Open dat file