Can't Download my Video via PyTube ending with mp3

Hello, does anyone know what I did wrong? Or why my video is downloaded in mp4 but NOT in mp3?

def converter(request): 
    if request.method == 'POST':
        link = request.POST['link']
        video = YouTube(link)
        format = request.POST['format']
        if format == "3":
            stream = video.streams.get_audio_only()
            download_folder = os.path.expanduser("~")+"/Downloads/"
            stream.download(download_folder)
            os.rename(stream.download + ".mp3")
            
        elif format == "4":
            stream = video.streams.get_highest_resolution()
            download_folder = os.path.expanduser("~")+"/Downloads/"
            stream.download(download_folder)
    

    return render(request, 'home/videoconverter.html',)