Putting on an end user hat I see no reason why AddOutputFilterByType shouldn't do exactly what it says it does.
I believe it has more to do with mod_proxy than the filter design. No one, at the time we added AddOutputFilterByType, wanted to rewrite mod_proxy to be knowledgeable about filters.
Ultimately, all that is needed is a call to ap_set_content_type() before any bytes are written to the client to get AddOutputFilterByType to work. Perhaps with the recent momentum behind mod_proxy work, someone could investigate that and get mod_proxy fixed. -- justin
