Upload image with ajax only works png

I trying to upload a picture (and more text data) using AJAX and it works... but only with png. Uummm... I only tried png and jpg, and jpg doesn´t work. I am not using a form. This is my javascript code:

if(window.FileReader)
{
    reader = new FileReader();
    reader.onloadend = function(e)
    {
        data["image"] =  e.target.result.replace('data:image/png;base64,', '');
        $.post('/notifications/simple_message', data, function(result){
           console.log(result);
        });
    };
    reader.readAsDataURL(img);
}   

And on the server side (using node) I have this:

if(req.body.image)
{
    var date = new Date();
    var buf = new Buffer(req.body.image, 'base64');
    image = req.session.user.name+'-'+date.getTime()+'.png';
    fs.writeFile('./public/images/users/'+image, buf);
}

I am using the png extension for the new file... but I tried using jpg and it didn´t work.

Any idea??

Thank you!!