docs: function createBodystructure(tree, options) {
      options = options || {};
  
      var walker = function(node) {
          switch((node.parsedHeader['content-type'] || {}).type) {
              case 'multipart':
                  return processMultipartNode(node, options);
              case 'text':
                  return processTextNode(node, options);
              case 'message':
                  if (!options.attachmentRFC822) {
                      return processRFC822Node(node, options);
                  }
                  return processAttachmentNode(node, options);
              default:
                  return processAttachmentNode(node, options);
          }
      };
      return walker(tree);
  }