[dba-Tech] Puzzling Attachments

I have tried to send pics to various friends, whose various email clients,
and I'm not sure who uses which. But several friends have replied that the
JPGs I send arrive as files called winmail.dat rather than Myfile.JPG. I
have no idea how to fix this issue, nor even an idea which email clients
create winmail.dat. I think hotmail is one of the guilty parties, but I
could be wrong about that.
Anyone know which clients create the attachment named winmail.dat? Is it
exactly the same attachment (JPG) that I sent? In that case a simple rename
would fix the issue. But I don't know.
What if I archive the JPG first? Presumably the result would still be named
winmail.dat, but it might be the archive file as it was sent.
Any suggestions?
TIA,
Arthur