extend the upload module

[Alexandr Gomoliako]

> I suspect that these extensions would be out of scope for this module,
> which seems to be more about making it easier to write your own form
> handler.

> Fundamentally, what you seem to want is for the handler to read a part
> of the POST request body and then decide whether to respond and close
> the connection, or accept the rest of the request body.
>
> I'm not sure how you'd go about that.

This is what nginx-perl is for.