[KNewFileMenu] Add extension to proposed filename

Summary:
Using "create new" text or html file, add the extension to the proposed
filename, this makes those plain text files work for other OS's e.g.
Windows (with Linux plain text files (.txt) don't usually have a extension
relying on mimetype detection); as for but HTML filenames they do have
extensions even on Linux.

Adapt the knewfilemenutest to the changes.

BUG: 61669

Test Plan: make && ctest; and using the create name text/html file menu in dolphin

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28302