Files in preferences.d dir should have extension .pref or non

Note that the files in the /etc/apt/preferences.d directory are parsed in alphanumeric ascending order and need to obey the following naming convention: The files have either no or "pref" as filename extension and only contain alphanumeric, hyphen (-), underscore (_) and period (.) characters. Otherwise APT will print a notice that it has ignored a file, unless that file matches a pattern in the Dir::Ignore-Files-Silently configuration list - in which case it will be silently ignored.