BUG: Prevent FreeSurfer overlay load from custom extensions

Revert change of allowing loading of FreeSurfer scalar overlays from custom extensions.
It interfered with the logic that determined what reader to use for loading model nodes.

Users may use composite file extensions that end with a standard extension.
For example, instead of lh.meanthickness_contrl these would all work:
lh.contrl.mean.thickness, lh.mean_contrl.thickness, lh_mean_contrl.thickness.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26070 3bd1e089-480b-0410-8dfb-8563597acbee