下面的示例添加了一个名为Buy Now的自定义媒体字段。此示例通过ajax将自定义字段的值保存在媒体覆盖屏幕以及媒体编辑屏幕（非ajax）上。

编辑：添加了临时检查和清理。

/**
 * Add custom field to media.
 */
add_filter( \'attachment_fields_to_edit\', \'wpse256463_attachment_fields\', 10, 2 );
function wpse256463_attachment_fields( $fields, $post ) {
    $meta = get_post_meta( $post->ID, \'buy_now\', true );

    $fields[\'buy_now\'] = [
      \'label\'        => __( \'Buy Now\', \'text-domain\' ),
      \'input\'        => \'text\',
      \'value\'        => $meta,
      \'show_in_edit\' => true,
      \'extra_rows\'   => [
        \'nonce\' => wp_nonce_field(
          \'update_attachment_buy_now\', // Action.
          \'nonce_attachment_buy_now\', // Nonce name.
          true, // Output referer?
          false // Echo?
        ),
      ],
    ];

    return $fields;
}

/**
 * Update custom field within media overlay (via ajax).
 */
add_action( \'wp_ajax_save-attachment-compat\', \'wpse256463_media_fields\', 0, 1 );
function wpse256463_media_fields() {
  $nonce = $_REQUEST[\'nonce_attachment_buy_now\'] ?? false;

  // Bail if the nonce check fails.
  if ( empty( $nonce ) || ! wp_verify_nonce( $nonce, \'update_attachment_buy_now\' ) ) {
    return;
  }

  // Bail if the post ID is empty.
  $post_id = intval( $_POST[\'id\'] );
  if ( empty( $post_id ) ) {
    return;
  }

  // Update the post.
  $meta = $_POST[\'attachments\'][ $post_id ][\'buy_now\'] ?? \'\';
  $meta = wp_kses_post( $meta );
  update_post_meta( $post_id, \'buy_now\', $meta );

  clean_post_cache( $post_id );
}

/**
 * Update media custom field from edit media page (non ajax).
 */
add_action( \'edit_attachment\', \'wpse256463_update_attachment_meta\', 1 );
function wpse256463_update_attachment_meta( $post_id ) {
  $nonce = $_REQUEST[\'nonce_attachment_buy_now\'] ?? false;

  // Bail if the nonce check fails.
  if ( empty( $nonce ) || ! wp_verify_nonce( $nonce, \'update_attachment_buy_now\' ) ) {
    return;
  }

  $buy_now = isset( $_POST[\'attachments\'][ $post_id ][\'buy_now\'] )
    ? wp_kses_post( $_POST[\'attachments\'][ $post_id ][\'buy_now\'] )
    : false;

  update_post_meta( $post_id, \'buy_now\', $buy_now );

  return;
}