Intercept Output From Default Handler With Another HttpHandler?

I have set up an HttpHandler for *css to do some simple parsing:

<handlers>
<add name="CssHandler" verb="*" path="*.css"
type="MyApp.CssProcessor,MyApp.Assembly"/>
</handlers>

All was well until I added a resource that loads a css file dynamically, e.g.

<link rel="stylesheet" type="text/css" href="/loader.ashx/module.resource.css" >

To my surprise, things went horribly wrong. The custom http handler intercepts this, but since it's designed to just load files from the file system, it doesn't work. I realize that, technically, it matches a pattern *.css but that seems an odd behaviour, since the actual resource being requested from the web server is *.ashx and the css is only after the file path, as a parameter.

Is it possible to make the filter for a handler only apply to the actual server resource name?

Alternatively (and actually I'd like to know how to do this anyway) -- what I would really rather be doing is intercepting the output from the default css handler. That is, rather than having all my own code to actually load files from the file system in my CSS handler, it seems it would be far simpler to just take the response from the default handler and filter it. Which would have worked properly in this situation.

Finally, in either case, I'd much rather be filtering on resource MIME type text/css rather than intercepting requests by name, since what I really want to do is filter any CSS (rather than anything that happens to be named "*.css"). how to do this?