All of that Rich Text information is stored in the winmail.dat file, and Outlook occasionally attaches regular file attachments in the winmail.dat file as well. Stop Outlook from Sending the Winmail.dat Attachment. The good news is you CAN stop Outlook from sending this attachment out to others who will have a lot of trouble opening it. Mar 09, 2015 · It's not available in Outlook (yet), but maybe someday. To use, click Insert > Attachments or OneDrive Files when you are composing a message. Select the file and click Next, then choose Share with OneDrive to insert a link to the file.