you can override this function to somehow react to an upload in progress.
Take note that parts of the CGI object is not yet initialized! Stuff from HTTP headers, including get, is usable. But, none of post is usable, and you cannot write here. That's why this method is const - mutating the object won't do much anyway.
My idea here was so you can output a progress bar or something to a cooperative client (see arsd.rtud for a potential helper)
The default is to do nothing. Subclass cgi and use the CustomCgiMain mixin to do something here.