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Thumbnails in Windows Explorer no longer show .rfa files after installing Revit 2013.
The computer still has Revit 2013 installed, and thumbnails worked before installing 2014.
It turned out to be more simple than I thought. Keep in mind that I have a Windows 10 system with a 64-bit operating system, x64-based processor. Not sure if that matters but here's the solution:
1. Go to your file explorer, locate your families folder that are not appearing
2. select all
3. left hand mouse click and select "Smart Sync"
4. Select "local"
Hope that helps.