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.......