Browsers don't recognize base64 svg if the mimetype is missing.
So we add the image mimetype to our flat odf export, which the xhtml
export uses and transforms it into an xhtml document.

Change-Id: I21aafdb97b4104e14e2d40abda73a526bb37041a
Reviewed-on: https://gerrit.libreoffice.org/40371Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>