It seemed to depend on the OS, that was my first thought too (and at first I downloaded it in Windows 10 using Firefox and got *.PNG’s), but your experiment shows that there must be another unknown factor, and so I tried it with my iPhone, my Android-Phone and also with my Android-VM via Chrome - in all cases I got *.kpp-files. 
I also thought it was because Google wanted to do something good for us and take work off our hands, but there seems to be another hidden reason for this behavior. I wonder if Google will roll some dice beforehand.
And then I tested with Opera, Chrome and Edge on Windows 10 and funnily got *.KPP’s. It seems to be a browser thing! Wow, that was weird.
Michelist