Illustrator document thumbnail previews not showing in finder window

I am trying to find a way to view Illustrator document thumbnails (CS,CS2,CS3 and CS4) in Finder windows. Other documents such as .tiffs, jpgs, etc and Word and .pdfs show but not Illustrator.
I used +show view options+ in the Finder, turning on and off show icon, show icon preview and show preview in column. The options seem to affect all other files but not Illustrator. All I get is the application icon but not the document thumbnail. Interestingly, I discovered if I manually delete the .ai extension the documents thumbnail appears, so I tried to turn off the +Show all file extensions+ in the Finder's Preferences. It changed other files but not Illustrator. Does anyone know how to accomplish this on a global basis without manually changing each files extension? It would allow me to the use of Quick Look on these files also. I did speak to Adobe and they thought it was an OS issue.