Generic logic to preserve custom files with
their correct ContentType. Standard default
file extensions with respective ContentType
preserved in [Content_Types].xml.

Change-Id: I651ed691e9a4745cd2cb4b3c4d4c5fd7287b66c2
Reviewed-on: https://gerrit.libreoffice.org/50856Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>