image upload class error in php

I write a php code for ajax image upload
when I don`t use class it works true,
but when I use it in class $_POST is true but $_File is empty:

client side(HTML Jquery):
<pre lang="HTML">
<div style="height:150px;width:97%;padding:10px 8px;">
               <form action="upload" method="POST" id="uploadform">
                  <input type="file" name="file"/>
                  <input type="submit" value="Upload"/>
                  <div id="loader" style="display:none;">
                   <center><img src="load.gif" /></center>
                  </div>
                  <div>
                   جزئیات :
                   <div id="onsuccessmsg" style="padding:15px;"></div>
                  </div>
               </form>
            </div>
                <script>
                $(document).ready(function(){
                 function onsuccess(response,status){
                  $("#loader").hide();
                  $("#onsuccessmsg").html("Status :<b>"+status+'</b><br><br>Response Data :<div id="msg" style="border:5px solid #CCC;padding:15px;">'+response+'</div>');
                 }
                  $("#uploadform").on('submit',function(){
                  $("#loader").show();
                  var options={
                   url     : $(this).attr("action"),
                   success : onsuccess
                  };
                  $(this).ajaxSubmit(options);
                 return false;
                 });
                });
            </script>

<?php
function getExtension($str) {$i=strrpos($str,".");if(!$i){return"";}$l=strlen($str)-$i;$ext=substr($str,$i+1,$l);return $ext;}
$formats = array("jpg", "png", "gif", "bmp", "jpeg", "PNG", "JPG", "JPEG", "GIF", "BMP");

if(isset($_POST) and $_SERVER['REQUEST_METHOD'] == "POST"){
 $name = $_FILES['file']['name'];
 $size = $_FILES['file']['size'];
 $tmp  = $_FILES['file']['tmp_name'];
 if(strlen($name)){
  $ext = getExtension($name);
  if(in_array($ext,$formats)){
   if($size<(1024*1024)){
    $imgn = time().".".$ext;
    if(move_uploaded_file($tmp, "folder address".$imgn)){
     echo "<br/>File Location : folder address".$imgn;
    }else{
     echo "Uploading Failed.";
    }
   }else{
    echo "Image File Size Max 1 MB";
   }
  }else{
   echo "Invalid Image file format.";
  }
 }else{
  echo "Please select an image.";
  exit;
 }
}
?>

<?php
class Upload
{
    function index()
    {
        #print_r($_POST);exit;
        $formats = array("jpg", "png", "gif", "bmp", "jpeg", "PNG", "jpg", "JPEG", "GIF", "BMP","pdf");

        if(isset($_POST) and $_SERVER['REQUEST_METHOD'] == "POST")
        {
            $name = $_FILES['file']['name'];
            $size = $_FILES['file']['size'];
            $tmp  = $_FILES['file']['tmp_name'];
            if(strlen($name)){
                $i=strrpos($name,".");
                if(!$i)
                {
                    $ext="";
                }
                $l=strlen($name)-$i;
                $ext=substr($name,$i+1,$l);
                if(in_array($ext,$formats)){
                    if($size<(1024*1024)){
                        $imgn = time().".".$ext;
                            if(move_uploaded_file($tmp, "folder address".$imgn)){
                                echo "<br/>File Location :folder address".$imgn;
                            }
                            else
                            {
                                echo "Uploading Failed.";
                            }
                    }
                    else
                    {
                        echo "Image File Size Max 1 MB";
                    }
                }
                else
                {
                    echo "Invalid Image file format.";
                }
            }
            else
            {
                echo "Please select an image.";
                exit;
            }
        }
    }
}
?>