Regression from 13596e70 (Don't pass ODF
version, just retrieve it directly, 2018-05-17), just checking the file
format explicitly like the PDF signing does fixes the problem.

Change-Id: I1ba7cf9096e01c6c5a2d3875b08fa3a66acae829
Reviewed-on: https://gerrit.libreoffice.org/56635
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>