I made some changes and fixes on the Plone add-on that drives the current LibreOffice templates website. This changes makes it possible for a site administrator to set allowed file extensions on the site within the template center content type (without the need to touch the source code).
I created a release of this Plone add-on and published it today on the Cheeseshop (https://pypi.org) (release 0.31):
https://pypi.org/project/tdf.templateuploadcenter/