title: AVAssetWriter invalid URL for writing
body: <p>I'm getting this error:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetWriter initWithURL:fileType:error:] invalid parameter not satisfying: [outputURL isFileURL]'
  </code></pre>
  
  <p>On this line:</p>
  
  <pre><code>AVAssetWriter * assetWriter = [[AVAssetWriter alloc] initWithURL:videoURL
                                                          fileType: AVFileTypeMPEG4
                                                             error: &amp;movieError];
  </code></pre>
  
  <p>Where videoURL logs as:</p>
  
  <pre><code>/var/mobile/Applications/A032EEA6-C83D-49DA-B118-E4E4B9F41C7F/Documents/videoForSegmentNumber1.mp4
  </code></pre>
  
  <p>the line before this is called.</p>
  
  <p>I've also tested against isFileURL myself and its returning NO. What is required to be a valid fileURL?</p>