This allows Windows/applications to pick OOXML editors (and thus
LibreOffice) when they lookup by content type, not by extension.

Change-Id: I0daca12f735035e6fc39484b5c788af37b81b575
Reviewed-on: https://gerrit.libreoffice.org/65563
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>