What is the correct way to save a file in Django to a specific location?

Question

Django

Hello! Please help me figure it out.
There is a data model Product. Each product has a category. There is also a form for creating a new record, where the user specifies a category and uploads a file.
The question is: how, depending on the selected category (assuming there are only two categories), to specify a specific download location for files outside the MEDIA_ROOT specified in the settings?
Structure:

my_site  <- основная директория, где хранится сам проект
my_site_files  < - Файлы нужно хранить за пределами проекта
      |____ category_1   <- файлы продукции с категорией 1 хранятся здесь
      |____ category_2   <- файлы продукции с категорией 2 хранятся здесь

models.py:

class Product(models.Model):
    file = models.FileField(
        max_length=255,
        blank=True,
        null=True,
        validators=[validate_file_extension]
    )

What I did:
To start, I created an environment variable (PRODUCT_FILE_PATH) with an absolute address to the my_site_files folder. Thought I would use this variable. Further as far as I understood it is necessary to use signals. I started like this, but to be honest I got confused:

@receiver(post_save, sender=Document)
def update_file_path(sender, instance, created, *args, **kwargs):
  if created:
    product_file=instance.file
    old_name=product_file.name
    if not old_name:
      return
    if instance.category==1:
      # Здесь сохранить путь для первой категории
    elif instance.category==2:
      # Здесь сохранить путь для первой категории
    instance.save()