title: Check if a file already exists before downloading it with Alamofire + suggestedDownloadDestination
body: <p>How can I check if a given file has been already downloaded before re-downloading it by using Alamofire? I'm using <code>suggestedDownloadDestination</code> so Alamofire will automatically choose the name of the file and save it in the choosen directory, for example the <code>.CachesDirectory</code>. The problem is that the value given by <code>suggestedDownloadDestination</code> is of type <code>DownloadFileDestination</code> which will return a <code>NSURL</code> only by calling him with the request's <code>response</code>, but in this way I could not ever know the file path without performing a request before.</p>
  
  <p>This is the code I currently use to download a file with Alamofire:</p>
  
  <pre><code>Alamofire.download(.GET, downloadLink, destination: destination).progress {
                      bytesRead, totalBytesRead, totalBytesExpectedToRead in
  
                      }.response {
                          request, response, data, error in
  
                          guard error == nil else {
                              return
                          }
  
                          // This will give me the file path but we're already in a Request!
                          print("\(destination(NSURL(string: "")!, response!))")
                  }
  </code></pre>
  
  <p>What am I missing?</p>