GWDVA optimisation switch: --dvafilter

In trying to keep the GWDVA running as optimal as possible I looked closer at the switch --dvafilter

TID 7017203 suggest filtering pdf and img files, but I thought if anything most PDF's should be indexed and graphic files would be better candidates for filtering as there is no text to search for. So a dva filter with "jpg,png,gif,img" would be worth doing.

The online GW docs are very brief on this filters use and recommendations.
What do you put in your --dvafilter?