/** Return distinct message for sent, upload progress, & response events */
private getEventMessage(event: HttpEvent, file: File) {
  switch (event.type) {
    case HttpEventType.Sent:
      return `Uploading file "${file.name}" of size ${file.size}.`;

    case HttpEventType.UploadProgress:
      // Compute and show the % done:
      const percentDone = Math.round(100 * event.loaded / event.total);
      return `File "${file.name}" is ${percentDone}% uploaded.`;

    case HttpEventType.Response:
      return `File "${file.name}" was completely uploaded!`;

    default:
      return `File "${file.name}" surprising upload event: ${event.type}.`;
  }
}