更新帖子时不保存Meta Box数据

Question

I am trying to add a meta box, following a tutorial. I have triple checked the code that it matches that of the tutorial (in which case, the meta box data IS saving), but on mine, it is not saving. This is my code.

<?php
/*
Plugin Name: Custom Post Meta Box
Plugin URI: http://acetronaut.com
Description: Demonstrates how to implement a custom posts meta box into 
wordpress
Version: 1.0.0
Author: Acetronaut
Author URI: http://acetronaut.com
License: GPL2
*/

function acetrnt_add_admin_styles() {
    wp_enqueue_style( 'acetrnt-admin', plugins_url( 'custom-post-meta-box/css/admin.css' ) );
}
add_action( 'admin_enqueue_scripts', 'acetrnt_add_admin_styles' );

function acetrnt_add_meta_box() {   
    add_meta_box(
        'acetrnt_audio',                // The ID for the meta box
        'Add MP3',                      //The title of the meta box
        'acetrnt_display_meta_box',     //The function for rendering the markup
        'post',                         // We'll only be displaying this on post pages
        'side',                         //Where the meta box should appear
        'core'                          // The priority of where the meta box whould be displayed
    );
}
add_action( 'add_meta_boxes', 'acetrnt_add_meta_box' );

function acetrnt_display_meta_box( $post ) {
    wp_nonce_field( plugin_basename( __FILE__ ), 'acetrnt_nonce_field' ); ?>

    <label id="mp3-title" for="mp3-title">Title of MP3</label>
    <input type="text" id="mp3-title" name="mp3-title" value="<?php echo esc_attr( get_post_meta( $post->ID, true ) ); ?>" placeholder="Your Song By Elton John" />

    <label id="mp3-file" for="mp3-file">MP3 File</label>
    <input type="file" id="mp3-file" name="mp3-file" value="" />

    <?php 
}

function acetrnt_save_meta_box_data( $post_id ) {
    if ( acetrnt_user_can_save( $post_id, 'acetrnt-nonce-field' ) ) {
        if ( isset( $_POST['mp3-title'] ) && 0 < count( strlen( trim( $_POST['mp3-title'] ) ) ) ) {
            $mp3_title = $_POST['mp3-title'];
            update_post_meta( $post_id, 'mp3-title', $mp3_title );
        }
    }
}
add_action( 'save_post', 'acetrnt_save_meta_box_data' );

function acetrnt_user_can_save( $post_id, $nonce ) {
    // Is this an autosave?
    $is_autosave = wp_is_post_autosave( $post_id );
    // Is this a revision?
    $is_revision = wp_is_post_revision( $post_id );
    // Is the nonce valid?
    $is_valid_nonce = ( isset( $_POST[ $nonce] ) && wp_verify_nonce( $_POST[ $nonce ], plugin_basename( __FILE__ ) ) );
    // Return true if the user is able to save the file
    return ! ( $is_autosave || $is_revision ) && $is_valid_nonce;
}
?>

I am banging my head against the wall as to the culprit. Is there anything in my code that is wrong? I have also tried dropping out of php and using regular html code, but that does not work.