Why am I receiving DAT files

I never had this issue until a few weeks ago.  I am receiving DAT files from a few people who routinely send me excel files.  They claim not to be doing anything different.  The files are the same version and in the same format.  When they send the files to my other emails (my work email) there doesn't appear to be any issues.  No one else receiving these emails are having any issues.

Can anyone tell me what is going on.