how to upload a file from body response?

gistfile1.js

var request = require('request');

var slack_token = process.env.SLACK_TOKEN;
var channel = process.env.SLACK_CHANNEL;

request.post({
    url: 'https://slack.com/api/auth.test',
    formData: {
        token: slack_token
    }
},
function(err, httpResponse, slackResBody) {
    if (err) {
        console.log('ERROR');
        console.log(err);
    } else {
        var slack_url = JSON.parse(slackResBody)["url"];
        console.log('slack url: ' + slack_url);
        var url = "https://www.google.com.ar/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png";
        request.get({
            url: url
        },
        function(err, res, body) {
            request.post({
                url: slack_url + '/api/files.upload',
                formData: {
                    channels: channel,
                    token: slack_token,
                    // How to upload without creating a file and using the body instead?
                    //
                    // The following will work:
                    //    var fs = require('fs');
                    //    fs.writeFileSync 'my-file.png' , body;
                    //    var readStream = s.createReadStream(tempFile);
                    // 
                    file: body // using 'readStream' here will work
                }
            },
            function(err, res, body) {
                if (err) {
                    console.log('ERROR');
                    console.log(err);
                } else {
                    console.log(res);
                }
            })
        });
    }
});