对于Swift，如果你想要你可以试试此框架

For Swift, If you want you can try with this framework

另外添加content-available ：你的aps中有1个

Also Add "content-available":1 in your aps

或者你可以尝试这样下载，

OR you can try downloading like this,

override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {

            self.contentHandler = contentHandler
            bestAttemptContent = (request.content.mutableCopy() as?UNMutableNotificationContent)

            bestAttemptContent?.title = request.content.title
            bestAttemptContent?.body = request.content.body

            guard let content = (request.content.mutableCopy() as? UNMutableNotificationContent) else {
                return failEarly()
            }

            guard let payload = content.userInfo["CustomData"] as? [String: Any] else {
                return failEarly()
            }

            guard let attachmentURL = payload["Attachement-url"] as? String else {
                return failEarly()
            }


            let identifierName = getIdentifierName(fileURL: attachmentURL)
            let tmpSubFolderName = ProcessInfo.processInfo.globallyUniqueString

            guard let imageData = NSData(contentsOf:NSURL(string: attachmentURL)! as URL) else { return failEarly() }

            guard let attachment = UNNotificationAttachment.create(imageFileIdentifier: identifierName, data: imageData, options: nil, tmpSubFolderName: tmpSubFolderName) else { return failEarly() }

            content.attachments = [attachment]
            contentHandler(content.copy() as! UNNotificationContent)
        }

    }


    func getIdentifierName(fileURL : String) -> String {
        var identifierName : String = "image.jpg"

        if !fileURL.isEmpty() {
            identifierName = "file.\((fileURL as NSString).lastPathComponent)"
        }

        return identifierName
    }

    func failEarly() {

        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            contentHandler(bestAttemptContent)
        }
    }

    extension UNNotificationAttachment {
        static func create(imageFileIdentifier: String, data: NSData, options: [NSObject : AnyObject]?, tmpSubFolderName : String) -> UNNotificationAttachment? {

            let fileManager = FileManager.default
            let tmpSubFolderName = ProcessInfo.processInfo.globallyUniqueString
            let fileURLPath      = NSURL(fileURLWithPath: NSTemporaryDirectory())
            let tmpSubFolderURL  = fileURLPath.appendingPathComponent(tmpSubFolderName, isDirectory: true)

            do {
                try fileManager.createDirectory(at: tmpSubFolderURL!, withIntermediateDirectories: true, attributes: nil)
                let fileURL = tmpSubFolderURL?.appendingPathComponent(imageFileIdentifier)
                try data.write(to: fileURL!, options: [])
                let imageAttachment = try UNNotificationAttachment.init(identifier: imageFileIdentifier, url: fileURL!, options: options)
                return imageAttachment
            } catch let error {
                print("error \(error)")
            }

            return nil
        }
    }