code: def get_thumbnail_format(self):
          
          if self.field.thumbnail_format:
              # Over-ride was given, use that instead.
              return self.field.thumbnail_format.lower()
          else:
              # Use the existing extension from the file.
              filename_split = self.name.rsplit('.', 1)
              return filename_split[-1]
docs: Determines the target thumbnail type either by looking for a format
          override specified at the model level, or by using the format the
          user uploaded.