code: async function onDataRead(filename, data) {
      log('file ' + path.basename(filename) + ' read');
      
      const ext = path.extname(filename).replace(/^\./, '');
      
      const optimizedData = await minify[ext](data);
      
      let b64Optimize;
      if (ext === 'css')
          [, b64Optimize] = await tryToCatch(minify.img, filename, optimizedData);
      
      return b64Optimize || optimizedData;
  }
docs: Processing of files
  @param fileData {name, data}