code: function pickFile(rootFolder, fileName) {
      var result = path.join(rootFolder, fileName);
      if (fs.existsSync(result)) {
          return result;
      }
  
      return undefined;
  }
docs: Pick a file, returns the absolute path if exists, undefined otherwise
  
  @param {String} rootFolder
  @param {String} fileName
  @return {String}