Happens on all PC’s at client site running 8.1
Does not happen on one client pc Running Windows 7 and Office 2010
Set up a Windows 8.1 machine with Office 2013 at our office, tested sending attachments to known email addresses who receive the attachments as winmail.dat files, all received the test email attachments fine. I tested both my email account and the clients email account, same results.
Some google searches suggest adding the attachment 3 different ways, drag and drop, attach with paperclip icon, and from within the office application, file, share, email, send as attachment .