django-cleanup automatically deletes old file for FileField, ImageField and subclasses, and it also deletes files on models instance deletion.
Warning! If you use transactions you may lose you files if transaction will rollback. If you are concerned about it you need other solution for old file deletion in your project.
Most django projects I've seen don't use transactions and this app is designed for such projects.
django-cleanup connects pre_save and post_delete signals to special functions(these functions delete old files) for each model which app is listed in INSTALLED_APPS above than 'django_cleanup'.
pip install django-cleanup
Add django_cleanup to settings.py
INSTALLED_APPS = (
...
'django_cleanup', # should go after your apps
)
django_cleanup should be placed after all your apps. (At least after those apps which need to remove files.)