When i save a photo with Compupic (and also with Fuji Finepix
software) as a jpg the resultant file appears in Windows explorer just
by its title and without any file extension e.g "Mac in front of
sphinx" Other software cannot see it in "Brouse" but just show an
empty folder.
The problem is easily solved by adding ".jpg" to make "Mac in front of sphinx.jpg"
Question: is there any way of adding ".jpg" to hundreds of files in
dozens of folders quicker than manually one-at-a-time?

Is the only reason you want to add '.jpg' to the files is so it shows
up in the windows tool bar?

If so, in windows explorer if you go 'tools, options, view' can can
tell it to display the full path and extension in the toolbar.

the easiest way i can think of is to get a command line
(start->run->"cmd") and from there change directory to the directory
where the files are with: "cd C:\Windows\yourfolder1\yourphotofolder"
(with your folder names as necessary), then use this command: "ren *
*.jpg"  that will rename everything(*) to its appropriate name with
.jpg appended to it.  Use the change directory command to go to your
next desired folder, rinse and repeat.  Good luck.