code: function getBasicFields(node, options) {
      var bodyType = node.parsedHeader['content-type'] && node.parsedHeader['content-type'].type || null,
          bodySubtype = node.parsedHeader['content-type'] && node.parsedHeader['content-type'].subtype || null,
          contentTransfer = node.parsedHeader['content-transfer-encoding'] || '7bit';
  
      return [
          // body type
          options.upperCaseKeys ? bodyType && bodyType.toUpperCase() || null : bodyType,
          // body subtype
          options.upperCaseKeys ? bodySubtype && bodySubtype.toUpperCase() || null : bodySubtype,
          // body parameter parenthesized list
          node.parsedHeader['content-type'] &&
              node.parsedHeader['content-type'].hasParams &&
              flatten(Object.keys(node.parsedHeader['content-type'].params).map(function(key) {
                  return [
                      options.upperCaseKeys ? key.toUpperCase() : key,
                      node.parsedHeader['content-type'].params[key]];
              })) || null,
          // body id
          node.parsedHeader['content-id'] || null,
          // body description
          node.parsedHeader['content-description'] || null,
          // body encoding
          options.upperCaseKeys ? contentTransfer && contentTransfer.toUpperCase() || '7bit' : contentTransfer,
          // body size
          node.size
      ];
  }
docs: Generates a list of basic fields any non-multipart part should have
  
  @param {Object} node A tree node of the parsed mime tree
  @param {Object} [options] Optional options object (see createBodystructure for details)
  @return {Array} A list of basic fields