title: Image from attachment from Local notification is not shown in UNNotificationContentExtension
body: <p>I've been working on rich notification experience which has been introduced in iOS10 and stuck with passing images as attachments to <code>UNNotificationContentExtension</code>.</p>
  
  <p>Here's my ContentExtension:</p>
  
  <pre><code>class NotificationViewController: UIViewController, UNNotificationContentExtension {
  
      @IBOutlet weak var attachmentImage: UIImageView!
  
      func didReceive(_ notification: UNNotification) {
  
          if let attachment = notification.request.content.attachments.first {
              if attachment.url.startAccessingSecurityScopedResource() {
                  attachmentImage.image = UIImage(contentsOfFile: attachment.url.path)
                  attachment.url.stopAccessingSecurityScopedResource()
              }
          }
      }
  }
  </code></pre>
  
  <p>As a tutorial, I've been following <a href="https://developer.apple.com/videos/play/wwdc2016/708/" rel="noreferrer">Advanced Notifications video from WWDC</a>.
  I've checked - <code>UIImage</code> I'm assigning to UIImageView:</p>
  
  <ul>
  <li>is <em>not</em> <code>nil</code></li>
  <li>has proper <code>CGSize</code> (191x191)</li>
  <li>attachment.url.path equals  <code>/var/mobile/Library/SpringBoard/PushStore/Attachments/&lt;bundle of app&gt;/&lt;...&gt;.png</code></li>
  </ul>
  
  <p><strong>Here's how I send local notification from the app</strong>:</p>
  
  <pre><code>    let content = UNMutableNotificationContent()
      content.title = "Sample title"
      content.body = "Sample body"
      content.categoryIdentifier = "myNotificationCategory"
  
      let attachement = try! UNNotificationAttachment(identifier: "image",
                                                                  url: Bundle.main.url(forResource: "cat", withExtension: "png")!,
                                                                  options: nil)
  
      content.attachments = [ attachement ]
  
      let request = UNNotificationRequest(identifier:requestIdentifier, content: content, trigger: nil)
      UNUserNotificationCenter.current().delegate = self
      UNUserNotificationCenter.current().add(request){(error) in
          if (error != nil){
          }
      }
  </code></pre>
  
  <p>"cat.png" is just a dummy resource I've added to proj.
  <a href="https://i.stack.imgur.com/6pzEv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/6pzEv.png" alt="enter image description here"></a></p>
  
  <p>As you can see, notification shows the pic, so I assume, that I'm sending it correctly, but in the expanded state(in <code>NotificationViewController</code>)  I've never succeed at showing the same image.</p>
  
  <p>What am I doing wrong?
  Thanks!</p>