NodeJs Formidable post form blocked script

i use nodejs and formidable to show progress bar of file from post form.

var formidable = require('formidable'),
    http = require('http'),
    util = require('util');
http.createServer(function(req, res) {
  if (req.url == '/test.html' && req.method.toLowerCase() == 'post') {
	  console.log("FILE UPLOAD")
    // parse a file upload 
    var form = new formidable.IncomingForm();
    form.parse(req, function(err, fields, files) {
      res.writeHead(200, {'content-type': 'text/plain'});
      res.write('received upload:\n\n');
      res.end(util.inspect({fields: fields, files: files}));
	  form.on('progress', function(bytesReceived, bytesExpected) {
		  console.log(bytesReceived+" / "+bytesExpected);
  // show a file upload form 
  res.writeHead(200, {'content-type': 'text/html'});
    '<form action="/test.html" enctype="multipart/form-data" method="post">'+
    '<input type="text" name="title"><br>'+
    '<input type="file" name="upload" multiple="multiple"><br>'+
    '<input type="submit" value="Upload">'+

In local, all is ok, console log bytesreceived et bytesexpected during send file. But in my server, console log is blocked until file upload is finished... Even for 'console.log("FILE UPLOAD") appear only after file finished... So i can't track progress event...

Perhaps it's a server behavior to configure, i'm on cent os 7 linux. Thank you for your help.