cache based on content type

I would try a switch:

when HTTP_RESPONSE {
   switch -glob [HTTP::header Content-Type] {
      *image/* {
         HTTP::header replace Cache-Control public,max-age=604800,must-revalidate,proxy-revalidate
      }
      text/css -
      application/x-javascript {
         HTTP::header replace Cache-Control private,max-age=604800,must-revalidate,proxy-revalidate
      }
      default {
         HTTP::header replace Cache-Control private
      }
   }
}

You could also combine the two similar cases merely using an "or":

...
} elseif { ( [HTTP::header Content-Type] equals "text/css" ) or ( [HTTP::header Content-Type] equals "application/x-javascript" ) } {
HTTP::header replace Cache-Control private,max-age=604800,must-revalidate,proxy-revalidate
} else {
...

However, this is slightly less efficient because the HTTP::header command still has to be evaluated multiple times.

HTH