code: 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);
  }
docs: Generates an object out of parsed mime tree, that can be
  serialized into a BODYSTRUCTURE string
  
  @param {Object} tree Parsed mime tree (see mimeparser.js for input)
  @param {Object} [options] Optional options object
  @param {Boolean} [options.contentLanguageString] If true, convert single element array to string for Content-Language
  @param {Boolean} [options.upperCaseKeys] If true, use only upper case key names
  @param {Boolean} [options.skipContentLocation] If true, do not include Content-Location in the output
  @param {Boolean} [options.body] If true, skip extension fields (needed for BODY)
  @param {Object} Object structure in the form of BODYSTRUCTURE