我正在尝试发送images推送通知，我已在应用程序委托中进行了通知注册，并且apns设备令牌正在正确生成。我也已经在服务分机中编写了如下代码：

import UserNotifications

class NotificationService: UNNotificationServiceExtension {

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

    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
self.contentHandler = contentHandler
bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)

// Get the custom data from the notification payload
if let notificationData = request.content.userInfo["data"] as? [String: String] {
    // Grab the attachment
    if let urlString = notificationData["attachment-url"], let fileUrl = URL(string: urlString) {
        // Download the attachment
        URLSession.shared.downloadTask(with: fileUrl) { (location, response, error) in
            if let location = location {
                // Move temporary file to remove .tmp extension
                let tmpDirectory = NSTemporaryDirectory()
                let tmpFile = "file://".appending(tmpDirectory).appending(fileUrl.lastPathComponent)
                let tmpUrl = URL(string: tmpFile)!
                try! FileManager.default.moveItem(at: location, to: tmpUrl)

                // Add the attachment to the notification content
                if let attachment = try? UNNotificationAttachment(identifier: "", url: tmpUrl) {
                    self.bestAttemptContent?.attachments = [attachment]
                }
            }
            // Serve the notification content
            self.contentHandler!(self.bestAttemptContent!)
            }.resume()
    }
}
}
}

。并且json中的有效负载如下

{
    "aps":
            {"sound":"default","alert":
                                        {"title":"iOS","body":"Hello Dude...."},
            "mutable-content": 1},
    "CustomData":
                    {"mType":"alert","m":"Hello Dude...."},
    "Attachement-url":"https://pusher.com/static_logos/320x320.png"
}

我收到 标题 和 消息， 但是 图像 未显示。请指导如何在推送通知中获取图片