code: function processFile() {
    const options = { collapseWhitespace: true };
    return (file, callback) => {
      if (file.meta.filePath.ext !== '.html') {
        async.nextTick(callback, null, file);
        return;
      }
  
      const data = htmlMinifier.minify(file.data, options);
      async.nextTick(callback, null, Object.assign({}, file, { data }));
    };
  }
docs: Process a single file.