Added a cavaet to the use of get_FOO_filename() and get_FOO_url(). This

constraint has always existed, but it's very hard to fix in the current code,
so better to work around it for now.

Refs #5619


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7999 bcc190cf-cafb-0310-a4f2-bffc1f526a37