BUG: #3014 fix failing MRB condition where . was missing from extension

This is a follow on fix for r22143 to handle the case where the filename
already has the correct extension.  Before this fix a situation
could arise where the filename would be something like ".nrrdnrrd" and
so it would fail to save.  Now it is correctly saved as ".nrrd".

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22146 3bd1e089-480b-0410-8dfb-8563597acbee