Commit 83b43ef2 (sw mail merge: add
support for the new 'writer' connectivity driver, 2017-07-18) fixed
this, with:

> If we are at it, also accept xlsx next to xls.

But there was no test for that part.

Change-Id: Iffbd4d21e52700d38dc0a83cb5540fa10c062944
Reviewed-on: https://gerrit.libreoffice.org/46818Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>