Indeed the best things in life are free.I am recently come across this open source software called Extension Changer.It is a small and simple program that can change the extension of multiple files in one go. Just drag and drop the file into the form or choose the new right-click option in the Windows Explorer context menu.This is a very useful program which can easily edit the extensions of many files.This can be used to repair files with unknown extensions and also hide files with confidential data.Suppose you want to prevent someone from accessing a .txt document.You can change the extension .txt to .jpeg ie. rendering the file inaccessible and corrupt unless the extension is corrected.The best thing about this program is that its completely open source and very lightweight.Here's the download link:--