code: function(file, enc, cb) {
        if (
          !file.isNull() &&
          !file.isDirectory() &&
          options.extensions.some(function(ext) {
            return path.extname(file.path).substr(1).toLowerCase() === ext;
          })
        ) {
          file = retina.tapFile(file, cb);
        } else {
          cb();
        }
        // Push only if file is returned by retina (otherwise it is dropped from stream)
        if (file) this.push(file);
      }
docs: Transform function filters image files