Upload XML into Codeignter 3

To uploading in codeigniter 3 and it is not accepting xml to send. It shows me an error that the selected file type is not allowed.

$config['upload_path']          = './uploads';
$config['allowed_types']        = 'xml';
$config['max_size']             = 0;
$config['max_width']            = 0;
$config['max_height']           = 0;
$config['encrypt_name']         = TRUE;

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

    if ( ! $this->upload->do_upload('nota_fiscal'))
    {
        echo $this->upload->display_errors();
    }

My mimes.php file looks like this:

'xml'   =>  array('application/xml', 'text/xml', 'text/plain'),