Ok, I've got a working thumbnail_frame.png image. I will work on it a little more and polish it up, but in the mean time, if you want to test it out just do the following.
Backup the original file using:
sudo mv /usr/share/pixmaps/nautilus/thumbnail_frame.png thumbnail_frame.png.bak
sudo mv thumbnailframe.png /usr/share/pixmaps/nautilus/thumbnail_frame.png
Restore to the original again:
If you need the original, I've uploaded it here:
sudo mv /usr/share/pixmaps/nautilus/thumbnail_frame.png thumbnailframe.png
sudo mv thumbnail_frame.png.bak /usr/share/pixmaps/nautilus/thumbnail_frame.png
Let me know how it works for you. Here is an actual screenshot of it working for me (click to view the full size)..
I'm gonna tweak the thumbnail_frame.png to lighten the shadow a bit, but I'll post it as well.