Custom Meta Box Wordpress

// add custom meta box "Product Description" for post type "Product"
add_action( 'add_meta_boxes', 'aim_product_description' );
function aim_product_description() {
    add_meta_box(
        'product_des',
        'Product Description',
        'aim_p_des_output_func',
        'product',
        'normal'
    );
}

function aim_p_des_output_func( $post ){
    wp_nonce_field( basename( __FILE__ ), 'aim_p_des_nonce' );
    $apd_meta_stored = get_post_meta( $post->ID );
?>
    <p>
        <label for="file_size" style="width: 60px;display:inline-block;">File Size</label>
        <input type="text" id="file_size" class="regular-text" name="_filesize"
        value="<?php if ( ! empty( $apd_meta_stored['_filesize'] ) ) {
                echo esc_attr ( $apd_meta_stored['_filesize'][0] );
             }?>" >
    </p>
    <p>
        <label for="file_type" style="width: 60px;display:inline-block;">File Type</label>
        <input type="text" id="file_type" class="regular-text" name="_filetype"
        value="<?php if ( ! empty( $apd_meta_stored['_filetype'] ) ) {
                echo esc_attr ( $apd_meta_stored['_filetype'][0] );
             }?>" >
    </p>
    <p>
        <label for="file_url" style="width: 60px;display:inline-block;">File Url</label>
        <input type="text" id="file_url" class="regular-text" name="_fileurl"
        value="<?php if ( ! empty( $apd_meta_stored['_fileurl'] ) ) {
                echo esc_attr ( $apd_meta_stored['_fileurl'][0] );
             }?>" >
    </p>
<?php
}


function apd_meta_save ( $post_id ) {
    // Checks save status
    $is_autosave = wp_is_post_autosave( $post_id );
    $is_revision = wp_is_post_revision( $post_id );
    $is_valid_nonce = ( isset( $_POST['aim_p_des_nonce'] ) && wp_verify_nonce( $_POST['aim_p_des_nonce'], basename( __FILE__ ) ) ) ? 'true' : 'false';

    // Exits script depending on save status
    if ( $is_autosave || $is_revision || !$is_valid_nonce ) {
        return;
    }

    if ( isset( $_POST['_filesize'] ) ) {
        update_post_meta( $post_id, '_filesize', sanitize_text_field( $_POST['_filesize'] ) );
    }
    if ( isset( $_POST['_filetype'] ) ) {
        update_post_meta( $post_id, '_filetype', sanitize_text_field( $_POST['_filetype'] ) );
    }
    if ( isset( $_POST['_fileurl'] ) ) {
        update_post_meta( $post_id, '_fileurl', sanitize_text_field( $_POST['_fileurl'] ) );
    }
}
add_action( 'save_post', 'apd_meta_save' );