I've updated my Theme Performance blog entry below on using "Expires" for Portal content to also force more Cache-Control directives on content already not tagged by Portal.
In researching this, I found that a lot of people think that you can use "FilesMatch" in the httpd.conf file to match content coming from Portal/WAS back to IHS thru the plugin for this purpose. It will not work! The reason is that "FilesMatch" will only match the file name of files stored on the IHS server itself. To match content coming back from Portal/WAS, you have to use "LocationMatch". See my example in my blog post below related to Cache Control.
Portal administration and performance
Matching: plugin X