On Windows 8, Kolourpaint uses the native Save As dialog. It defaults to PPM files, however, and does not remember the last used setting, which usually is PNG or JPG. If it's not possible to remember it for the native dialog, at least it should default to PNG. Reproducible: Always
Not kolourpaint specific. It's handled by kdelibs KFileDialog
Works for me in KDE Frameworks 5.45.