If I want to rename a file - i.e. text document or windows document or jpeg file, I get message as "if ou change filename extension, then the file may become unstable, are you sure you want to change it. If I accept and change it then it shows and image icon and won't open. Previously if I wanted to rename a text document I typed in the new name and .txt was added automatically by the computer. Now the only way to change a name is to a)type in the new name with say .txt or .jpeg at the end or b)open the file and save as
It is as Diodorus Siculus says. If you have selected "Hide file extensions for known file types", Windows will assume the extension. If the option is unselected, Windows leaves you in control of the extension. If you are already in Windows Explorer, you get the option via "View - Folder Options - View"