Hi, I've found the answer on Polish Android forum:
Laptop z Windows 10 nie widzi Galaxy S6. - Pomoc techniczna - Forum Android.com.pl
The answer is to use different cable
SImple as that.
I've had the same problem on my S6 + win10 + original S6 cable. After switching to another USB cable (one from Kindle I belive) I could finally copy files between windows and S6.
Laptop z Windows 10 nie widzi Galaxy S6. - Pomoc techniczna - Forum Android.com.pl
The answer is to use different cable

I've had the same problem on my S6 + win10 + original S6 cable. After switching to another USB cable (one from Kindle I belive) I could finally copy files between windows and S6.