Question about sending form via post

I hope this is the right place...

I'm using "HttpWebRequest" in order to send a web form via "post".
but I have a problem, the site get a component like this one: <input type="file" name="the_file"....>
how can I add a file to the post data? I mean adding it to the post string: "domain_lang=en&action=upload&the_file=........"

thanks.


...