Moved valid extensions to their own file

extensions.py will contain vaild input and output extensions
Revert "Moved valid extensions to their own file"

This reverts commit 3f4335756d0dfc5dd10749970687b37b2d314b2a.
Revert "Revert "Moved valid extensions to their own file""

This reverts commit 088b90f8d5cc539aa6e5d98a2e1bb38f0cc2a12a.
removed extensions

they now reside in extensions.py
updated postConversion to use new extensions-based check