import UserNotifications

class NotificationService: UNNotificationServiceExtension {

    var contentHandler: ((UNNotificationContent) -> Void)?
    var bestAttemptContent: UNMutableNotificationContent?

    // リッチ通知のプッシュ通知を受け取るとdidReceiveが呼ばれる
    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
        self.contentHandler = contentHandler
        bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)

        if let notificationData = request.content.userInfo["notification-data"] as? [String: String] {
            if let urlString = notificationData["attachment-url"], let fileURL = URL(string: urlString) {
                URLSession.shared.downloadTask(with: fileURL) { (location, response, error) in
                    if let location = location {
                        // メディアファイルをダウンロードしてtmpに保存
                        let tmpFile = "file://".appending(NSTemporaryDirectory()).appending(fileURL.lastPathComponent)
                        let tmpUrl = URL(string: tmpFile)!
                        try? FileManager.default.moveItem(at: location, to: tmpUrl)

                        if let attachment = try? UNNotificationAttachment(identifier: "hoge", url: tmpUrl, options: nil) {
                            // メディアをtmp
                            self.bestAttemptContent?.attachments = [attachment]
                        } else {
                            // メディアの添付に失敗
                            self.bestAttemptContent?.body = "メディア添付に失敗した時の通知の文言"
                        }
                    }
                    contentHandler(self.bestAttemptContent!)
                    }.resume()
            } else {
                // URLの取得に失敗したときの処理
                contentHandler(self.bestAttemptContent!)
            }
        } else {
            // JSONペイロードからメディアのデータが取得できなかったときの処理
            contentHandler(self.bestAttemptContent!)
        }
    }

    // didReceiveの処理が30秒を超えると呼ばれる。
    override func serviceExtensionTimeWillExpire() {
        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            bestAttemptContent.body = "メディアのダウンロードがTimeoutになったときの通知の文言"
            contentHandler(bestAttemptContent)
        }
    }
}

{
  "aps": {
    "alert": "メディア付きリッチ通知だよ",
    "mutable-content": 1
  },
  "notification-data": {
    "attachment-url": "メディアのURL"
  }
}

let options: [AnyHashable: Any]  = [
    UNNotificationAttachmentOptionsTypeHintKey : kUTTypeJPEG,
    UNNotificationAttachmentOptionsThumbnailClippingRectKey : CGRect(x: 0, y: 0, width: 1, height: 1).dictionaryRepresentation,
    UNNotificationAttachmentOptionsThumbnailTimeKey:2,
    UNNotificationAttachmentOptionsThumbnailHiddenKey : true
] as [String : Any]

if let attachment = try? UNNotificationAttachment(identifier: "hoge", url: tmpUrl, options: options)
{ ...