[1.10.x] Fixed #26644 -- Allowed wrapping NamedTemporaryFile with File.

914c72be 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 1b407050 from master