Jifty::Web::Form::Field::Upload - File upload field
An input field that renders using <input type="file" />.
The argument value that the action recieves from this field via "argument_value" in Jifty::Action will be a filehandle,
which can be read in the usual ways.
Renders the file upload widget.
The 'value', rendered, is empty so that BLOBs and the like don't get streamed to the browser.
Add 'upload' to the rest of the classes
