fixed :)

server.js

var http = require('http');
var fs = require('fs');
var ip = '127.0.0.1';
var port = 2000;
var www = process.cwd();
var mimetypes = {'html': 'text/html', 'js': 'text/javascript', 'css': 'text/css', 'png': 'image/png', 'svg': 'image/svg'};
var path = require('path');

http.createServer(function (req, res) {
    fs.realpath(path.join(www, req.url), function(err, realpath){
        if (err || realpath.substr(0, www.length+1) != www+'/') {
            res.writeHead(404, {'Content-Type': 'text/plain'});
            return res.end('No such file exists, sorry!');
        }
        
        var filename = realpath.split('/').pop();
        var extension = (filename.split('.').length > 1)
            ? filename.split('.').pop()
            : 'txt';
        var mimetype = (mimetypes[extension] === undefined)
            ? 'application/octet-stream'
            : mimetypes[extension];
        res.writeHead(200, {'Content-Type': mimetype});
        fs.createReadStream(realpath).pipe(res); //herein lies the problem
        // commented this out :)
        // res.end();
    });
}).listen(port, ip);
console.log('Serving '+www+' at http://'+ip+':'+port);