- Go to Start> Run
- Type regedit and hit enter
- Now navigate to HKEY_CURRENT_USER > Software> Microsoft> Windows> Current Version> Explorer
- Create two new DWORD values: ThumbnailSize and ThumbnailQuality .
- Modify ThumbnailSize with value data 256 and Base as decimal.
- Modify ThumbnailQuality with value data 100 and Base as decimal.
- Click OK and restart pc for changes to take effect.
http://www.winmatrix.com/forums/index.php?/topic/19751-get-thumbnails-of-more-file-types-text-document-and-3rd-party-file/