Implements a renaming policy that adds increasing integers to the body of
any file that collides. For example, if foo.gif is being uploaded and a
file by the same name already exists, this logic will rename the upload
foo1.gif. A second upload by the same name would be foo2.gif.
Note that for safety the rename() method creates a zero-length file with
the chosen name to act as a marker that the name is taken even before the
upload starts writing the bytes.
1.1, 2002/11/05, making thread safe with createNewFile(), 1.0, 2002/04/30, initial revision, thanks to Yoonjung Lee
for this idea