How to write code correctly?

Question

Django

class VideoDoc(models.Model):
  name_video = models.CharField("Название", max_length=150, null=True, blank=True)
  poster = models.ImageField("изображение", upload_to = 'product/videos/posters/', null=True, blank=True)
  video = models.FileField("Видео", upload_to = "product/videos/", null=True, blank=True)
  name_doc = models.CharField("Название", max_length=150, null=True, blank=True)
  files = models.FileField("Документы", upload_to = "product/documents/", null=True, blank=True)
  video_accept = models.BooleanField("Это видео")
  doc_accept = models.BooleanField("Это документ")
  doc_format = models.ForeignKey(Format, verbose_name='Формат файла', on_delete=models.DO_NOTHING)

  """function for demo visit"""
  pdf = VideoDoc()

  def demo():
    filename, file_extension = os.path.splitext(pdf.files)
    if file_extension == ".docx":
      pass
    elif file_extension == ".pdf":
      images = convert_from_path(pdf.files, fmt="png")
    else:
      pass
    return images

  class Demo(models.Model):
    demo_img = models.ImageField("Демо изображение", upload_to="product/documents/demo/", default=demo())

    def __str__(self):
      return f'{self.demo_img} - {self.VideoDoc().name_doc}'

    class Meta:
      verbose_name = 'Демо просмотр'
      verbose_name_plural = "Демо просмотр"

  class Demo_docx(models.Model):
    demo_img = models.ImageField("Демо изображение", upload_to="product/documents/demo_doc/")

    def __str__(self):
      return f'{self.demo_img} - {self.VideoDoc().name_doc}'

    class Meta:
      verbose_name = 'Демо для док файлов'
      verbose_name_plural = "Демо для док файлов"
  demo_doc = models.ManyToManyField(Demo, verbose_name='Демо', null=True, blank=True)
  demoForDoc = models.ManyToManyField(Demo_docx, verbose_name="Демо для док файлов", null=True, blank=True)


  def __str__(self):
    return f'{self.name_video} - {self.name_doc}'

  class Meta:
    verbose_name = 'Видео и документ'
    verbose_name_plural = "Видео и документы"

What should be the result: When you enter a file in the files field, the file should be passed to the demo function and this function will convert the file into an image and all these images should be written to the demo_img field in the Demo class (Not to be confused with Demo_docx)