Otherwise the UNO API user can only see this image has a replacement,
but not possible to say if it's an SVG or a PDF image.

Change-Id: Ibde7915e02620acecbbb237dc3b333382d9c784a
Reviewed-on: https://gerrit.libreoffice.org/37827Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>