I found it:

http://www.acousticwebdesign.net/web-design/creating-custom-meta-boxes-in-wordpress/

将附件上载到帖子/页面时，它们将成为该帖子/页面的子元素。

您可以在自定义字段中提供一个下拉列表，列出当前帖子/页面上传到上传器的所有上传文件。这是我用来显示文件列表的代码，用户可以选择哪一个是平面图图像：

      <select name="chb_homes_for_sale_specifics_floor_plan" style="width:100%;">
        <option value="" <?php selected($custom["chb_homes_for_sale_specifics_floor_plan"][0], ""); ?>>No Floor Plan</option>
        <?php
        $args = array(
            \'numberposts\'     => -1,
            \'orderby\'         => \'menu_order\',
            \'order\'           => \'ASC\',
            \'post_type\'       => \'attachment\',
            \'post_parent\'     => $post->ID,
            \'post_mime_type\' => \'image\'
        );
        $image = get_posts($args);
        if($image) {
            foreach($image as $key => $data) : ?>

                <option value="<?php echo $data->ID; ?>" <?php selected($custom["chb_homes_for_sale_specifics_floor_plan"][0], $data->ID); ?>><?php echo basename ( get_attached_file( $data->ID ) ); ?></option>

            <?php endforeach;
        }
        ?>
      </select>