Newly installed Devuan Chimaera so probably using the current Debian binaries. Gwenview version is 20.12.4
JPEG files are opened with no problem but trying to save as JPEG throws an "unsupported image format" dialog. This happens irrespective of .jpg or .jpeg being given as the extension. Saving as PNG and other formats works OK. If this were a general problem I'd expect it to come up on a search so I expect it's something like a missing library or a configuration error. Does anyone have any ideas?

Ian

KolourPaint has the same issue. It looks very much like a library problem.
Coincidentally? selecting JPEG format on the save menu sets the default file extension to .jpe i.e. .jpeg truncated to 3 letters.
Krita doesn't exhibit either of these behaviours.