I noticed the other day that vista wasn’t showing any thumbnails for 3gp files. I searched around and came across a registry file (.reg) which adds thumbnails to explorer for a large number of media files. An excerpt of the file can be seen here:
The only bit of this which is of any interest is the bolded part. This is the part which specifies the file extension. You can find a reg file which ‘fixes’ all main media types below.
This image shows the before and after icons displayed in explorer:
Download it here
Use this at your own risk, i take no responsibility for any damage/problems that occur as a result of using this file.