// Rename the |file| to "foo.bar", but don't clobber "foo.bar" if it exists
var newFile = file.parent.clone();
newFile.append("foo.bar");
if (!newFile.exists())
  file.moveTo(null, "foo.bar");
else {
  newFile.createUnique(nsIFile.NORMAL_FILE_TYPE, PERMS_FILE);
  file.moveTo(null, newFile.leafName);
}