image uploader is not working in CI

This is my view page

<?php echo form_open_multipart('admin/item/uploadImage'); ?>
<input type="file" name="file" id="file" />
<td><?php echo form_submit('submit', 'Save', 'class="btn btn-primary"');?>

and This is my controller page :

class item extends Admin_Controller
{


public function __construct ()
{
    parent::__construct();
    $this->load->model('item_m');
}

public function index ()
{
    // Fetch all items
    $this->data['items'] = $this->item_m->get();
    // Load view
    $this->data['subview'] = 'admin/item/index';
    $this->load->view('admin/_layout_main', $this->data);


}




function uploadImage()

{

   $config['upload_path']   =   "uploads/";

   $config['allowed_types'] =   "gif|jpg|jpeg|png"; 

   $config['max_size']      =   "5000";

   $config['max_width']     =   "1907";

   $config['max_height']    =   "1280";

   $this->upload->initialize($config);
   $this->load->library('upload',$config);

   if(!$this->upload->do_upload($_POST['file']))

   {

       echo $this->upload->display_errors();

   }

   else

   {

       $finfo=$this->upload->data();

       $data['uploadInfo'] = $finfo;

       $data['thumbnail_name'] = $finfo['raw_name']. '_thumb' .$finfo['file_ext']; 

     //  $this->load->view('upload_success',$data);

       // You can view content of the $finfo with the code block below

       /*echo '<pre>';

       print_r($finfo);

       echo '</pre>';*/

   }

     //  $this->load->view('upload_success',$data);

       // You can view content of the $finfo with the code block below

       /*echo '<pre>';

       print_r($finfo);

       echo '</pre>';*/



}}

am getting error like this

A PHP Error was encountered
Severity: Notice
Message: Undefined index: file
Filename: admin/item.php
Line Number: 47
You did not select a file to upload.

please help me file data is not passing to controller page? i have tried to call doupload image from another function also but it is not working also.

The problem is with this statement :

if(!$this->upload->do_upload($_POST['file']))

Here you have to specify only parameter name, like :

if(!$this->upload->do_upload('file'))