code: function custom(options, onfile, done) {
  
     if ((!options) || (typeof(options) == 'function'))
        throw new Error("please provide some options");
  
     if (!options.fields)
        throw new Error("please provide some fields");
  
     return resolveList(builder.files, options, [], onfile, done);
  }
docs: lets you specifiy all the fields