Timeline Express插件

Question

我正在使用Wordpress的时间线快速插件。我正在尝试添加自定义图标，但它无法正常工作。我正在使用它：https://gist.github.com/EvanHerman/6bbc8de82f34b4cb3c5c但该字段未显示。

<?php
/*
Plugin Name: Timeline Express Custom Icon Filter Test
*/
add_filter( 'timeline-express-custom-icon-html', 'pn_timeline_express_custom_icon_html_test', 10, 3 );
function pn_timeline_express_custom_icon_html_test( $html, $post, $timeline_express_options ) {
    $custom_png_icon = get_post_meta( $post->ID, '_custom_png_icon', true );

    if ( !$custom_png_icon ) {
        return $html;
    } else {
        $image_html = '<img class="custom-image" src="' .$custom_png_icon . '" />';
    }
    if ( empty( $image_html) ) {
        return $html;
    }
    // capture custom image HTML for the icon
    ob_start();
    if ( $timeline_express_options['read-more-visibility'] != 0 ) { ?>
        <a class="cd-timeline-icon-link" href="<?php echo get_the_permalink( $post->ID ); ?>">
            <div class="cd-timeline-img cd-picture cd-timeline-png">
                <?php echo $image_html; ?>
            </div> <!-- cd-timeline-img -->
        </a>
    <?php } else { ?>
        <div class="cd-timeline-img cd-picture cd-timeline-png">
            <?php echo $image_html; ?>
        </div> <!-- cd-timeline-img -->
    <?php }
    $html = ob_get_contents();
    ob_end_clean();
    return $html;
}
add_filter( 'timeline_express_custom_fields' , 'add_custom_timeline_express_field' );
function add_custom_timeline_express_field( $custom_fields ) {
    $custom_fields = array(
        array(
            'name' => __( 'Example Text Field', 'timeline-express' ),
            'desc' => __( 'this is an example user defined text field.', 'timeline-express' ),
            'id'   => '_custom_png_icon',
            'type' => 'file',
        ),
    );
    return $custom_fields;
}
add_action( 'wp_footer', 'pn_timeline_express_custom_icon_html_footer_css' );
function pn_timeline_express_custom_icon_html_footer_css() {
    ?>
    <style>
        #primary .cd-timeline-img {
            border-radius: 0;
            box-shadow: none;
        }
        #primary .cd-timeline-img.cd-picture {
            background: none;
            border: none;
        }
        #primary .cd-timeline-img img {
            width: 60px;
            height: 60px;
            margin-left: -30px;
            margin-top: -30px;
            height: auto;
        }
    </style>
    <?php
}