Well, i unchecked it and all it did is it added .exe to executables, .txt to text files, etc. It didnt affect JAR extensions whatsoever...
I was more thinking that you had the extensions for .png/.jpg hidden,
so through explorer you couldn't tell whether they were lower/upper case.
The result being that only when they were jarred would:
a) the case of the extension become visible, and
b) a (negative) effect seen at runtime.