ENH: remove doubled extenions before saving

Some nodes when they are read in by the node readers end up with names that
include the extension, such as vol.vp. This change ensures that the
known extension will not be added to the end of the name to create a file
name with a doubled extension, such as vol.vp.vp.
Fixed a typo in a variable name.

Issue 0003956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24584 3bd1e089-480b-0410-8dfb-8563597acbee