914c72be2abb1c6dd860cb9279beaa66409ae1b2 introduced a regression that
causes saving a NamedTemporaryFile in a FileField to raise a
SuspiciousFileOperation. To remedy this, if a File has an absolute
path as a filename, use only the basename as the filename.
Backport of 1b407050dd53e56686fdd3e168f8cac4f9be8306 from master