Make it Modular

module.exports = function filter(dirname, ext, callback){
  // pass the array through the callback and then bam!
  fs.readdir(dirname, function(err, data){
    if(err){
      return callback(err);
    }

    var datum = [];

    for(i = 0; i<data.length;i++){
      if(path.extname(data[i]) == '.'+ext){
        datum.push(data[i]);
      }
    }

    callback(null, datum);

  });
}