title: AVURLAsset can't get mp3 duration in documents directory, but it works well with mp3 in app bundle
body: <p>I have two mp3 files. One is in app bundle, the other is in user's Documents directory. I want to get the duration of mp3 file.</p>
  
  <pre><code>AVURLAsset* audioAsset = [AVURLAsset URLAssetWithURL:[NSURL URLWithString:newPath] options:nil];
  [audioAsset loadValuesAsynchronouslyForKeys:@[@"duration"] completionHandler:^{
      CMTime audioDuration = audioAsset.duration;
      float audioDurationSeconds = CMTimeGetSeconds(audioDuration);
  
      NSLog(@"duration:%f",audioDurationSeconds);
  }];
  </code></pre>
  
  <p>Those codes work well with the  mp3 file in app bundle, but it doesn't work with the mp3 in Documents directory which only log "duration:0.000000". Why?</p>