node.js / image upload using express / transfer not complete

I'm trying to upload an image using ajax / node.js and express 3 (included Formidable)

The transfert is ok but not complete ... It missing a part of the image as you can see on this screenshot (the dark area)

This is the server post route code :

var form = req.form;

form.uploadDir = __dirname + '/client/tmp';
form.maxFieldsSize = 20 * 1024 * 1024;
form.keepExtensions = true;

form.parse(req, function(err, fields, files) {
    if (err) return console.log(err); 

    console.log('ok !');
});

EDIT

The weight of the uploaded image is bigger than original ... example : I try to upload a 80ko image, and the uploaded file have a weight of 140ko (and is not complete)

EDIT 2

If I use

form.on('file', function(name, file) {
});

Instead of

form.parse ...

It works, but don't know why ...

Any help would be very appreciated