[mod_python] AddOutputFilterByType

I'm running a PSP script which outputs a PNG image.  The PSP script sets 
the req.content_type to image/png but its still going through a mod_python 
filter I have setup with AddOutputFilterByType.  The AddOutputFilterByType 
is setup for two types, text/html and text/plain.  Does changing 
req.content_type not update the Apache info before it gets to the filtering 
stage?  Can this be fixed?

-Dan
mod_python: 3.1.2b
apache: 2.0.48
python: 2.3.2
linux: 2.4