Register flat file mime types on OS X

Change-Id: I505703b2f6890586e05e6c2146d9b37774f101d0
Reviewed-on: https://gerrit.libreoffice.org/11957
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>