1:-Goto control panel and choose folder option.
2:-When open folder option click View tab.
3:-click radio button on the hidden files and
folder.
4:-click “Show hidden files and folders” and then
uncheck “hide extension unknown files types”.
5:-and press ok Or apply like you can change
any file extension.
6:-After this process example for changing
extension.
Exa:-
1:-Select any file and rename with any extension.
2:-after this windows message click ok.
3:-you are use this file remember before extension.
And send any file/hide easily.......