docs: function addAttachment(obj_list) {
      for (let key in obj_list) {
        if (typeof(obj_list[key]) === 'string' &&
          regexAttach.test(obj_list[key])) {
          const binaryKey = obj_list[key];
          const replacement = getAttachment(binaryKey);
          if (replacement !== null) obj_list[key] = replacement;
        } else if (typeof(obj_list[key]) === 'object') {
          // arrays are also 'object' with this test.
          addAttachment(obj_list[key]);
        }
      }
    }