I tried this:
I selected the generic icon of the image file
In the menu, I selected "Open with"
My computer was set to open with: Capture NX 2.app (default)
I changed the default to: Preview.app
The generic icon instantly changed to a thumbnail of the image.
Then I changed "Open with" back to my original default and so far it seems to be working.
I did this both with my own generic icons, and with the generic icon I downloaded from Fredsson's DropBox.
I just did this a few minutes ago, so I can't vouch for how long this fix will last. Maybe the problem lies not with Apple but with Nikon. I have recently updated Nikon Capture NX 2, Nikon Transfer, and View NX2. I had had the problem with photofiles downloaded from a newly purchased Nikon P7100.