Why notification service extension is not firing on iOS 14?

You have to modify the UNNotificationRequest content part. Below is the code snippet which I have used for my notification extension.

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)
        
        if let bestAttemptContent = bestAttemptContent {
            var urlString:String? = nil
            if let urlImageString = request.content.userInfo["urlImageString"] as? String {
                urlString = urlImageString
            }
            // You can set what ever title you want to set
            bestAttemptContent.title = "Your custom tile goes here"
            if urlString != nil, let fileUrl = URL(string: urlString!) {
                print("fileUrl: \(fileUrl)")
                
                guard let imageData = NSData(contentsOf: fileUrl) else {
                    contentHandler(bestAttemptContent)
                    return
                }
                guard let attachment = UNNotificationAttachment.saveImageToDisk(fileIdentifier: "image.jpg", data: imageData, options: nil) else {
                    print("error in UNNotificationAttachment.saveImageToDisk()")
                    contentHandler(bestAttemptContent)
                    return
                }
                
                bestAttemptContent.attachments = [ attachment ]
            }
            
            contentHandler(bestAttemptContent)
        }
    }
    
    override func serviceExtensionTimeWillExpire() {
        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            contentHandler(bestAttemptContent)
        }
    }

}

@available(iOSApplicationExtension 10.0, *)
extension UNNotificationAttachment {
    
    static func saveImageToDisk(fileIdentifier: String, data: NSData, options: [NSObject : AnyObject]?) -> UNNotificationAttachment? {
        let fileManager = FileManager.default
        let folderName = ProcessInfo.processInfo.globallyUniqueString
        let folderURL = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(folderName, isDirectory: true)
        
        do {
            try fileManager.createDirectory(at: folderURL!, withIntermediateDirectories: true, attributes: nil)
            let fileURL = folderURL?.appendingPathComponent(fileIdentifier)
            try data.write(to: fileURL!, options: [])
            let attachment = try UNNotificationAttachment(identifier: fileIdentifier, url: fileURL!, options: options)
            return attachment
        } catch let error {
            print("error \(error)")
        }
        
        return nil
    }
}

The JSON payload which I have used

{
  "aps": {
    "alert": {
      "title": "title",
      "body": "Your message Here"
    },
    "mutable-content": 1
  },
  "urlImageString": "https://cdn.pixabay.com/photo/2015/04/23/22/00/tree-736885__340.jpg"
}

with your code repo the output which I am getting is as

If you want to debug the Notification Service extension then you have to select the Notification Service Extension scheme from the drop down and then your breakpoints will work for you

Update

I had submitted Techinical support Incident for this issue. I had performed extensive R & D on the notification service extension. implemented it according to apple docs in a vanila project. Our configuration and everything seemed to be correct. So i requested apple technical support on the issue and got this reply.

We have determined that you are seeing this issue due to a bug in iOS. Please submit a complete bug report regarding this issue using Feedback Assistant (https://feedbackassistant.apple.com). For more information on Feedback Assistant, please visit https://developer.apple.com/bug-reporting/.

Please include all the information you provided me once again in the bug report (you can attach the logs directly to the bug report). Please make sure to follow the instructions at https://developer.apple.com/bug-reporting/profiles-and-logs/ and follow the instructions for APNS (Apple Push Notification Service) to install a logging profile on your device, and make sure it is installed before reproducing the problem.

Important: In Feedback Assistant, when filing bug reports about an API/SDK on macOS, iOS, watchOS or tvOS, please select an appropriate API/SDK area (after "Something else not in this list”).

So, i am going to submit a bug report to apple. i will update this answer once got reply on the bug report.