Until I saw this thread I was not aware of the problem.
I have just finished searching the forum on the developer's site (
https://chevereto.com/community/ ) and I found a reference that the extensions were changed to jpeg after the upgrade to version 4 to avoid problems relating to the jpeg extension and to avoid having two naming conventions for what are the same types of files.
Images uploaded before the change are not effected.
To me this is an error because the jpg file extension is the most common, furthermore the permited extensions include both types. I will report this to the developer as a problem that should be a user choice and, in the mean time, investigate the possibility to override this behaviour.
If it is causing a problem in individual cases I should be able to change the extention back to jpg manually.