在底部链接的Trac记录单上，有一个解决方案可以使其正常工作

function _save_attachment_url($post, $attachment) {
    if ( isset($attachment[\'url\']) ) 
        update_post_meta( $post[\'ID\'], \'_wp_attachment_url\', esc_url_raw($attachment[\'url\']) ); 
    return $post;
}
add_filter(\'attachment_fields_to_save\', \'_save_attachment_url\', 10, 2);

function _replace_attachment_url($form_fields, $post) {
    if ( isset($form_fields[\'url\'][\'html\']) ) {
        $url = get_post_meta( $post->ID, \'_wp_attachment_url\', true );
        if ( ! empty($url) )
            $form_fields[\'url\'][\'html\'] = preg_replace( "/value=\'.*?\'/", "value=\'$url\'", $form_fields[\'url\'][\'html\'] );
    }
    return $form_fields;
}
add_filter(\'attachment_fields_to_edit\', \'_replace_attachment_url\', 10, 2);

要获取它，您只需要附件ID，您可以通过以下方式获取：

get_post_meta( $post->ID, \'_wp_attachment_url\', true );