Mime filter

I'm not sure this is the best group for this, but I
haven't found another:

I wrote a mime filter, loosely based on the MS
mimefilt sample. It all works fine except that the
filter "text/html" usually doesn't work online. It
works for offline webpages, and occasionally works
online, but usually doesn't work online. I've tried
IE security settings, file extension fiddling...I can't
find the problem. And I know that it's not a caching
issue. Yet the mime filter is never called at all with
most online pages.