Updating clientCache in web config not changing expires header

I have a site running on iis7.5 where, through Firebug, I can see that my static content (css, js and images) have an expiration in their header consistently set to one day in the future. I find no where this is set, but I just came on recently to the project.

I want to make the expiration further in the future, so added a section in t开发者_如何学Che web.config:

<system.webServer>
   <staticContent>
     <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="100.00:00:00" />
   </staticContent> 
</system.webServer> 

After restarting the site, this had no affect on the expiration date of these items.

My question is, what can override the clientCache setting, imposing a different expiration date?

Thanks,

Matt