node.js - How to upload file using a post request in the node http library?

Question

From the Node.js http library docs:

http.request() returns an instance of the http.ClientRequest class. 
The ClientRequest instance is a writable stream. If one needs to upload a file 
with a POST request, then write to the ClientRequest object.

However I'm unsure how to leverage this in my current code:

var post_data = querystring.stringify({
    api_key: fax.api_key,
    api_secret: fax.api_secret_key,
    to: fax.fax_number,
    filename: ""
});

var options = {
    host: p_url.hostname.toString(),
    path: p_url.path.toString(),
    method: 'POST',
    headers: {
          'Content-Type': 'application/x-www-form-urlencoded',
          'Content-Length': post_data.length
    }
};

var postReq = http.request(options, function(res) {
    res.setEncoding('utf8');
    res.on('data', function (chunk) {
          console.log('Response: ' + chunk);
    });
});

postReq.write(post_data);
postReq.end();