Is uploaded_file a valid parameter when invoking Feed Validator via Web Service API

Currently I'm using the feed validator via the web service in the following
manner.
Http Get http://validator.w3.org/feed/check.cgi?output=soap12&rawdata="", in
which
I append the feed data as the rawdata.  However I often get a 414
Request-URI Too Large.

Now the Doc explicitly outlines use of the "url", "rawdata" & "output"
parameters and
obliquely mentions the possible use of the "uploaded_file" parameter.  But
it's not clear
to me exacly how to use it. Any help would be appreciated.

Thanks in advance.
Paul Dick