Pass down mimetype to SvXMLGraphicHelper

Change-Id: I9c81c06d2a1d6168704440094081e99d0bcbbff9
Reviewed-on: https://gerrit.libreoffice.org/41893Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>