Squid: Parent proxy only for html and images

I’ve got a parent proxy that does compression on html and images but it has a cap of XXmb per day so I dont’ want to send everything to it…I want to continue to download direct rpm’s…zip’s etc…in fact I pretty much only want to send *.htm *.html *.jpg, *.jpeg and *.png traffic to the parent proxy…I already have squid running as a transparent proxy for my lan so how can I modify it to use the parent proxy only for those extensions?