title: Copying file from temp url to documents directory always throws. NSURLSessionDownloadTask
body: <p>I'm downloading a file using NSURLSessionDownloadTask which is working great. As I get my image or video file into a temp directory. But I need to move it to a permanent URL in order to put it into photos library. I'm using <code>NSFileManager</code> 's <code>copyItemAtURL:</code> without any success. Any reason it would throw? Perhaps file type is not compatible with documents directory? </p>
  
  <pre><code>let directory : String = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0]
  
  
  func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didFinishDownloadingToURL location: NSURL) {
  
      if let fileName = self.file.zOrigFileName {
          let destinationPath = self.directory.stringByAppendingString("/\(fileName)")
  
          if let destinationURL = NSURL(string: destinationPath) {
  
              let fileManager = NSFileManager.defaultManager()
  
              //IF file with same name exists delete it before copying new file
              if fileAlreadyExistsAtURL(destinationURL) {
                  do {
                      try fileManager.removeItemAtURL(destinationURL)
                  } catch {
                      print("Error Removing Item At \(destinationURL.path)")
                  }
              }
  
              do {
                  try fileManager.copyItemAtURL(location, toURL: destinationURL)
                  self.saveURLToPhotosLibrary(destinationURL)
              } catch {
                  //This is line always printing. my try statement always throwing.
                  print("Error Copying Item from \(location.path) to \(destinationURL.path)")
              }
          }
      }
  }
  </code></pre>
  
  <p>Here is the print statement. For security I'm replacing app bundle id from documents directory with <code>$(AppId)</code></p>
  
  <blockquote>
    <p>Error Copying Item from
    Optional("/private/var/mobile/Containers/Data/Application/E8D9C365-15D2-40BD-B0B5-A000BEDA9F00/Library/Caches/com.apple.nsurlsessiond/Downloads/$(AppID)/CFNetworkDownload_CK3G3Z.tmp")
    to
    Optional("/var/mobile/Containers/Data/Application/E8D9C365-15D2-40BD-B0B5-A000BEDA9F00/Documents/shortMovie.mov")</p>
  </blockquote>