[mod_python] Is it possible to stop file uploading?

Hello,
Can anyone please answer these questions?

1.When a user uploads file, using FORM ( POST method), is there a  possibility  to stop 
uploading, if the file extension  is not among those  allowed? 

2.
Is there a good/recommended way how to find out the file extension?
I use a handler that can find the extension( using  util.FieldStorage(req))
but it does not stop uploading in progress but uploads the whole file first.

Thank you for help
Lad.