Hi, I have a Lenovo W530 laptop. It was running Fedora 25, everything was okay.
However, today I upgraded to Fedora 26, and now I have some strange issues with the display brightness. If I press the keyboard shortcut to adjust brightness, the corresponding window appears, showing a progressbar with the brightness value. I can change that value, but, however, the actual screen brightness is not changed.
It must have something to do with the Nvidia graphics chip which is used in that laptop.

If I use the command

echo 100 >> /sys/class/backlight/intel_backlight/brightness

I can adjust the brightness, but this is no permanent solution :-)

If no one here has a better idea you can always put that command in a plain text file, chmod +x it, and set it as a startup script in GNOME Tweak tool

no, this is an useless solution because I need to increase or decrease the brightness, which works only with variable parameters for that command ;-)

when I run xbacklight, I get

$ xbacklight
No outputs have backlight property

Then you should be able to set custom keyboard shortcuts that issue the command to increase or decrease the brightness using the Cinnamon settings