It was returning a lowercased suffix because it was extracting it from
the pattern (*.txt) rather than from the filename ("README.TXT").
This broke expectations on the application side, since this method is
documented to return a suffix from the given filename.
Ref: https://bugs.kde.org/show_bug.cgi?id=402388
Change-Id: I7dae13db31280249d3f592fa9592c4067804e22d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>