Nexus 7 tablet won't show files when connected to pc
Connect your Nexus 7 to the PC, then on the android screen swipe down the icons from the top left side of the screen. One of the boxes shown will say "USB for Charging". Tap that box and select "Transfer files (MTP)". Now your Nexus files will show up on the PC as they used to do before the upgrade. 

BTW, you have to do this every time you connect to the PC.