Javascript function does not summarize $ _POST of PHP

I'm implementing a progress bar in a simple file upload script. I have the three files that are giving me a headache, Upload.php where the upload script is located, script.js where the script that updates the progress bar is located, and finally index.php where the upload form is.

The upload panel is a form that sends with the $_POST the trigger to the PHP file, and it always worked fine, after I implemented this script ( script.php ) began to give problem, the progress bar works, but the upload script in PHP is triggered.

index.php

<form action="" method="post" enctype="multipart/form-data">
     <input type="file" name="pic" />
     <!-- ........ -->
<button type="submit" name="btn-upload">Upload file</button> <!-- botão de upload -->

<div class="progress">
    <div class="bar"></div >
    <div class="percent">0%</div >
</div>

<!--scripts include-->

<!-- jQuery Library-->
<script type="text/javascript" src="js/jquery.min.js"></script>

<!-- jQuery Form Plug in -->
<script type="text/javascript" src="js/jquery.form.min.js"></script>

<!-- our main javascript file -->
<script type="text/javascript" src="js/script.js"></script>

<script type="text/javascript" src="js/demo.js"></script>

script.js

$(document).ready(function() {
    /* variables */
    var percent = $('.percent');
    var bar = $('.bar');

    /* submit form with ajax request */
    $('form').ajaxForm({

        /* set data type json */
        dataType:  'json',

        /* reset before submitting */
        beforeSend: function() {
            bar.width('0%');
            percent.html('0%');
        },

        /* progress bar call back*/
        uploadProgress: function(event, position, total, percentComplete) {
            var pVel = percentComplete + '%';
            bar.width(pVel);
            percent.html(pVel);
        },

    });
});

upload.php

(View complete file)

// muitas funções que uso na função abaixo
//
if(isset($_POST['btn-upload']))
{
    $original = $_FILES['pic']['name'];
    $array = explode('.', $original);

So, by summary, the problem is caused when I click the upload button, the progress bar walks, but the file is not sent, because the PHP upload function is not called.

  

I've tried to put a echo 'debug'; in the file upload.php to see if the function was called at least, but nothing appeared on the page.