Sorry for the delay, but in the meantime that laptop broke and I switched to another Macbook Pro which is supported and runs Catalina.
If I have for example opened a PNG file and if I go to “Save As…” and choose JPEG, then the extension is always “.jpeg”. Even if I change it, next time when I open a PNG file and try to “Save As…” as JPEG, the extension is always “.jpeg”
In Preview.app it hides the extension by default, but when I go to “Get Info” on the file saved by Preview, I see that the extension is “.jpg”