[BUGFIX] Fix use of isDot() in Cache\Filebackend

This inherits from Ib2a21f8607f464901b81c1950d112f639cbe29ba but the
changes to the temporary filename were already implemented.

Change-Id: I17454615f4c8d9a052bf0c1546fd138462e75b20
Related: #33621
Releases: master, 2.1