Hey, it looks like there is limitation with the SDK returning the video_bytes and mime_type, which I have escalated to the engineering team. I checked, and even if it returns None, I am still able to generate the video.
for n, generated_video in enumerate(operation.response.generated_videos):
client.files.download(file=generated_video.video)
generated_video.video.save(f"video{n}.mp4")