Every week I send an e-mail update to members of my club (not a photo
club). As usual, I included a few jpegs in today's e-mail. My address
is in the mailing list, and I was shocked to see that every jpeg had
been converted to a much larger file titled "unknown". None of my
software would open the images. I finally succeeded by adding a ".jpg"
suffix to the file name, but that hardly helps the others. My main
question is, why would this happen? I have literally sent thousands of
jpegs over the years, and hundreds of these weekly updates, and afaik
this is a first. Ideas?

I'm using a Macintosh G4 with up-to-date software, afaik!

Bob