How to stop ‘Winmail.dat’ attachments?

Since early May I have been receiving attachments as ‘webmail.dat’ files instead of a pdf or whatever type of file it is supposed to be.

I have checked to ensure my outlook is not set to send as ‘rich text format’. But I still only get Winmail.dat.

could this have been caused by the most recent iOS update? Or is there a way to fix this on my end?