code: function processFile(rootPath, preprocessor) {
    return (file, callback) => {
      if (preprocessor === StylePreprocessor.LESS) {
        const options = { paths: [rootPath], syncImport: true };
        less.render(file.data, options, (error, output) => {
          if (error) {
            console.error(`${file.path}: Could not render the LESS styles`);
            async.nextTick(callback, error);
            return;
          }
  
          const data = output.css;
          async.nextTick(callback, null, Object.assign({}, file, { data }));
        });
      } else {
        async.nextTick(callback, null, file);
      }
    };
  }
docs: Process a single file.