XSLT filter not invoked with HttpProxyServlet

Hi,

I've configured XsltFilter so that it should be filtering all content in my
webapp. It's configured unconditional to process all mime-types and
url-pattern is "/*". The xsl-transformation works correctly with my test
HelloWorld servlet, but the filter is not invoked at all when the
HttpProxyServlet is called.
Any ideas what should I do to get it working?

Thanks,
-Toni