Is winmail.dat file a virus? Why does this happen?
Below is a link to a post from Microsoft as to why this happens and what you can do to stop this from happening in future
Does the winmail.dat file have any content?
Not all winmail.dat files have content. Right-Click the winmail.dat and open in notepad. The first 2 lines of text will give you an idea if it contains any relevant data (eg: IPM.Microsoft Mail.Note)
How to open a winmail.dat file?
Found a simple and easy to use website that helps you extract the contents online