Once plugged in to PC, pull down your notification tray on the tablet. You will see a notification that says " USB for charging".
Tap it, more options will appear. Choose the one that says "for transferring media".
You will now see the files in windows file explorer.
You have to do this every time you want to transfer files.
It's the way Android 6 works.
Tablet now natively supports USB otg, so you can use a flash drive with it also.