NotificationsService.swift

override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
    func failEarly() {
      print("fail")
      contentHandler(request.content)
    }
      
    guard let content = (request.content.mutableCopy() as? UNMutableNotificationContent) else {
      return failEarly()
    }
      
    guard let attachmentURL = content.userInfo["attachement-url"] as? String else {
      return failEarly()
    }

    do {
      let imageData = try Data(contentsOf: URL(string: attachmentURL)!)
      
      guard let attachment = UNNotificationAttachment.create(imageFileIdentifier: "image.png", data: imageData, options: nil) else { return failEarly() }        
      content.attachments = [attachment]
      contentHandler(content.copy() as! UNNotificationContent)
        
    } catch {
  }
}

extension UNNotificationAttachment {
  /// Save the image to disk
  static func create(imageFileIdentifier: String, data: Data, options: [NSObject : AnyObject]?) -> UNNotificationAttachment? {
    let fileManager = FileManager.default
    let tmpSubFolderName = ProcessInfo.processInfo.globallyUniqueString
    let tmpSubFolderURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(tmpSubFolderName, isDirectory: true)
    
    do {
      try fileManager.createDirectory(at: tmpSubFolderURL, withIntermediateDirectories: true, attributes: nil)
      
      let fileURL = tmpSubFolderURL.appendingPathComponent(imageFileIdentifier)
      
      try data.write(to: fileURL)
      
      let imageAttachment = try UNNotificationAttachment.init(identifier: imageFileIdentifier, url: fileURL, options: options)
      
      return imageAttachment
    } catch let error {
      print(error)
    }
    
    return nil
  }
}