Generally, Outlook can sometimes reformat attachments to 'winmail.dat'. 

This is most often when the email client is sending out emails that contain Rich Text such as blog, italics, etc. 

The formatting is stored in the winmail.dat file. The recipient email clients may store that data as an attachment. Other normal file attachments are also stored as winmail.dat files as well. 

Below is a quick guide on how to stop this from happening. 

  1. Open up your Outlook email client.

  2. Click on Filelocated in the upper toolbar.

  3. Select the Optionssetting.

  4. From here, select the Mailcategory.

  5. Now, under Compose messages, you want to ensure the HTML or Plain Text option is selected for the Compose messages in this format.

  6. Next, underMessage format, set When sending messages in Rich text format to Internet recipients to Convert to HTML format or Plain Test format

  7. Click OK to finish.