There are files out there, on your hard drive, that don’t have extensions. This means there is no default application to open these files with. Being a geek, however, you want to see what’s inside.
Normally, you’d double click the file, and be presented with a dialog that asks what app should open the selected file. With the registry hack shown, you can bypass that step, and open any file without an extension with Notepad.
[HKEY_CLASSES_ROOT\*\shell]
[HKEY_CLASSES_ROOT\*\shell\open]
@="Open With Notepad"
[HKEY_CLASSES_ROOT\*\shell\open\command]
@="notepad.exe %1"
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.]
"Application"="Notepad"