[ticket/12211] Do not run attachment file names twice through htmlspecialchars

Upload filenames are already processed via htmlspecialchars in the
type_cast_helper of the new request class. There is no need to run it through
htmlspecialchars() again in the filespec class.

PHPBB3-12211
Merge remote-tracking branch 'marc/ticket/12211' into develop-ascraeus

* marc/ticket/12211:
  [ticket/12211] Do not run attachment file names twice through htmlspecialchars