title: QLThumbnailGenerator doesn't generate the thumbnail
body: <p>I tried using QLThumbnailGenerator in Simulator/Device for iOS and iPadOS, but it does not work.
  I'am able to only obtain a standard empty thumbnail but not the rich icon from my files from documents directory.</p>
  
  <p>Some progress with files in sandbox but nothing useful.</p>
  
  <p>Do you make it work?
  Maybe something with the permission is wrong...but what? From my app I am able to list files, and read (open) them.</p>
  
  <pre><code>@IBAction func generateDidSelect(_ sender: Any) {  
  
      let docDir = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true).last  
      print("DOC: \(docDir)")  
  
      let absPath = URL(fileURLWithPath: docDir ?? "").appendingPathComponent("flowers.png").absoluteString  
      //[[NSBundle mainBundle]URLForResource:@"flowers" withExtension:@"png"];  
      let fileURL = URL(fileURLWithPath: absPath)  
      fileURL.startAccessingSecurityScopedResource()  
  
      let size = CGSize(width: 200, height: 200)  
      let scale = UIScreen.main.scale  
  
      let isFile = fileURL.isFile()  
      let exists = FileManager.default.fileExists(atPath: fileURL.path)  
      print("isFILE? \(isFile ? "YES" : "NO") exists? \(exists ? "YES" : "NO") \nFILE: \(fileURL)")  
  
      let request = QLThumbnailGenerationRequest(fileAtURL: fileURL, size: size, scale: scale, representationTypes: QLThumbnailGenerationRequestRepresentationTypeAll)  
      //request.iconMode = YES;  
  
      QLThumbnailGenerator.shared().generateRepresentations(for: request, updateHandler: { thumbnail, type, error i  
  
          DispatchQueue.main.async(execute: {  
  
              print(String(format: "*** TYPE: %ld ****", Int(type)))  
              let uiimage = thumbnail?.uiImage  
              let cgImage = thumbnail?.cgImage  
  
              if let uiimage = uiimage {  
                  print("uiimage: \(uiimage)")  
              }  
              if let cgImage = cgImage {  
                  print("cgImage: \(cgImage)")  
             }  
  
              if uiimage != nil {  
                  self.thumbnailImageView.image = uiimage  
              }  
  
              if error != nil {  
                  if let error = error {  
                      print("ERROR: \(error)")  
                  }  
                  //self.thumbnailImageView.image = [UIImage imageWithContentsOfFile:fileURL.path]; // test read, works  
              }  
          })  
      })  
  
  }  
  
  </code></pre>
  
  <p>Then I tried with an image into the bundle.</p>
  
  <p>Getting the file url with:</p>
  
  <pre><code>Bundle.main.url(forResource: "flowers", withExtension: "png")  
  </code></pre>
  
  <p>and it magically works! ...but no with <code>fileURLWithPath</code> method.</p>
  
  <p>But, accessing the same identical file uploaded via iTunes into the Documents directory of the app i get a read error.</p>
  
  <pre><code>2019-10-01 12:41:27.167091+0200 test_thumb_obj[618:57118] DOC: /var/mobile/Containers/Data/Application/BE4A5950-5D24-4620-A1FE-B837222E8B64/Documents  
  2019-10-01 12:41:27.196739+0200 test_thumb_obj[618:57118] isFILE? YES exists? YES   
  FILE: file:///var/mobile/Containers/Data/Application/BE4A5950-5D24-4620-A1FE-B837222E8B64/Documents/flowers.png  
  2019-10-01 12:41:27.233546+0200 test_thumb_obj[618:57118] *** TYPE: 0 ****  
  2019-10-01 12:41:27.233788+0200 test_thumb_obj[618:57118] uiimage:   
  2019-10-01 12:41:27.233858+0200 test_thumb_obj[618:57118] cgImage:  (DP)  
    &lt; (kCGColorSpaceDeviceRGB)&gt;  
    width = 493, height = 640, bpc = 8, bpp = 32, row bytes = 1984   
    kCGImageAlphaPremultipliedFirst | kCGImageByteOrder32Little  | kCGImagePixelFormatPacked   
    is mask? No, has masking color? No, has soft mask? No, has matte? No, should interpolate? Yes  
  2019-10-01 12:41:27.234761+0200 test_thumb_obj[618:57118] *** TYPE: 1 ****  
  2019-10-01 12:41:27.234836+0200 test_thumb_obj[618:57118] uiimage: (null)  
  2019-10-01 12:41:27.234865+0200 test_thumb_obj[618:57118] cgImage: (null)  
  2019-10-01 12:41:27.234943+0200 test_thumb_obj[618:57118] ERROR: Error Domain=QLThumbnailErrorDomain Code=2 "No cached thumbnail"  
  2019-10-01 12:41:27.262228+0200 test_thumb_obj[618:57118] *** TYPE: 2 ****  
  2019-10-01 12:41:27.262317+0200 test_thumb_obj[618:57118] uiimage: (null)  
  2019-10-01 12:41:27.262349+0200 test_thumb_obj[618:57118] cgImage: (null)  
  2019-10-01 12:41:27.262452+0200 test_thumb_obj[618:57118] ERROR: Error Domain=QLThumbnailErrorDomain Code=0 "Could not generate a thum  
  bnail" UserInfo={NSUnderlyingError=0x281676940 {Error Domain=NSCocoaErrorDomain Code=256 "The file couldn’t be opened."}}  
  </code></pre>
  
  <p>The only image a get i TYPE = 0, the white empty one.</p>
  
  <p>And, as before, on simulator nothing good....</p>
  
  <pre><code>Error Domain=NSPOSIXErrorDomain Code=22 "couldn't issue sandbox extension com.apple.app-sandbox.read for...  
  </code></pre>
  
  <p>Some test i missed to make it working?</p>