wordpress上传徽标不起作用

I follow up the sitepoint tutorial to create logo uploader for my option page but the image uploader not working correctly.

here the functions.php:

function logo_display()
{
    ?>
        <input type="file" name="logo" /> 
        <?php echo get_option('logo'); ?>
   <?php
}

function handle_logo_upload()
{
    if(!empty($_FILES["demo-file"]["tmp_name"]))
    {
        $urls = wp_handle_upload($_FILES["logo"], array('test_form' => FALSE));
        $temp = $urls["url"];
        return $temp;   
    }

    return $option;
}

function display_theme_panel_fields()
{
    add_settings_section("section", "All Settings", null, "theme-options");

    add_settings_field("logo", "Logo", "logo_display", "theme-options", "section");  

    register_setting("section", "logo", "handle_logo_upload");
}

add_action("admin_init", "display_theme_panel_fields");

if you view source, you will prob see the enctype is not set to enctype="multipart/form-data". You could do something like below to change the enctype

function change_form_enc() {
        echo "<script type='text/javascript'>
                  jQuery(document).ready(function(){
                      jQuery('form').attr('enctype','multipart/form-data');
                  });
              </script>";
}



if ( is_admin() ) {

    add_action('admin_head', 'change_form_enc');
}

function logo_display()
{
    ?>
        <input type="file" id="logo" name="logo" /> 
        <?php echo get_option('logo'); ?>
   <?php
}

function handle_logo_upload()
{
    if(!empty($_FILES["logo"]["tmp_name"]))
    {
        $urls = wp_handle_upload($_FILES["logo"], array('test_form' => FALSE));
        if ($urls["error"]) 
        {
            return $urls["error"];
        }
        $temp = $urls["url"];
        return $temp;   
    } 
    return get_option('logo');
}