Movies and PDFs thumbnail previews not working

I had this problem a few weeks after I updated to Lion. I used to be able to play movies and music and turn PDF pages from the icon thumbnail itself by clicking a small play button on the movie or music thumbnail, or flip through a PDF document using small directional buttons appearing on the PDF thumbnail itself without tapping the spacebar for Quick Look, or actually opening the file itself. Somehow, I lost this functionality now without knowing how or why. It was pretty useful when you want to hear audio in the background without using any apps (iTunes, Quicktime, etc.) Has anyone encountered the same issue before?

OK, I finally sorted out the problem. It seems that thumbnail previews will only work if the default icon size is at least 64x64. I think I might have deleted the finder preferences .plist file by mistake which reverted the icon size to the system defaults, and led to this problem.

By the way, here is the article which helped me solve this issue.