Please confirm whether user is using mac or windows? 


Attachment format changing to winmail.dat for some mails. 

https://answers.microsoft.com/en-us/outlook_com/forum/all/why-i-am-getting-email-attachments-as-winmaildat/0f882a97-6c7e-4a80-96ba-4c0cf3e72dfa


Apple and mac don't support Outlook Rich Text Format (RTF)  it convert attachment into .dat when it received at apple or mac client.
https://www.transip.eu/knowledgebase/entry/99-get-winmaildat-file-attachment-email/#:~:text=dat%20file%20as%20an%20attachment%20in%20my%20email%3F&text=If%20you%20receive%20an%20email,message%20also%20uses%20Microsoft%20Outlook. 

Please follow below steps.
https://support.microsoft.com/en-gb/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722

Change message format Outlook Rich Text Format (RTF)  to plain text or html.
follow below link
https://support.microsoft.com/en-gb/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722 

https://learn.microsoft.com/en-us/troubleshoot/outlook/message-body/how-tnef-affects-email-messages