[1.6.x] Removed a doc reference to the deprecated `mimetype` kwarg.
[1.6.x] Removed a doc reference to the deprecated `mimetype` kwarg.

Backport of 61ed959235 from master

redhat-openstack/django,felixjimenez/django,django-nonrel/django,felixjimenez/django,felixjimenez/django,redhat-openstack/django,django-nonrel/django,redhat-openstack/django,felixjimenez/django,django-nonrel/django,django-nonrel/django,redhat-openstack/django