Fixed #15590 -- Documented how the path of a FileField can be changed.

    file name used on disk cannot be relied on until after the model has been

    saved.

For example, you can change the file name by setting the file's

:attr:`~django.core.files.File.name` to a path relative to the file storage's

location (:setting:`MEDIA_ROOT` if you are using the default

:class:`~django.core.files.storage.FileSystemStorage`)::

    >>> import os

    >>> from django.conf import settings

    >>> initial_path = car.photo.path

    >>> car.photo.name = 'cars/chevy_ii.jpg'

    >>> new_path = settings.MEDIA_ROOT + car.photo.name

    >>> # Move the file on the filesystem

    >>> os.rename(initial_path, new_path)

    >>> car.save()

    >>> car.photo.path

    '/media/cars/chevy_ii.jpg'

    >>> car.photo.path == new_path

    True

The ``File`` object

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