Fix regression in outputFileExtension

We have to use a QMap (or some other container) which keeps the order
(sorted by extension) of the entries because the order incidentally
puts "gpg" before "pgp", so that "gpg" is preferred over "pgp" when
iterating over the entries looking for an entry matching the
classification.

• Fixes-commit: 2ba6334dc4ce706a758c42106d1f1d14b51f0aec
• GnuPG-bug-id: T6726