title: Why the Extension is "momd" but not "xcdatamodel" when search the path for the Model file
body: <p>I create a Master-Detail application using Core Data. And there is  a simple code to 
  get the path for Model file:</p>
  
  <pre><code>NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Model" withExtension:@"momd"];
  </code></pre>
  
  <p>I have no idea why the extension isn't "xcdatamodel" but "momd". </p>
  
  <p>Does anyone know the answer?</p>