code: function processFile(extension) {
    return (file, callback) => {
      let filePath;
      if (file.meta !== undefined && file.meta.filePath !== undefined) {
        const ext = extension === null
          ? file.meta.filePath.ext
          : extension;
        filePath = path.format(Object.assign({}, file.meta.filePath, { ext }));
      } else {
        filePath = extension === undefined
          ? file.path
          : `${trimExtension(file.path)}${extension}`;
      }
  
      async.nextTick(callback, null, Object.assign({}, file, { path: filePath }));
    };
  }
docs: Process a single file.