By default file extensions are not displayed in windows, because file extensions for known file types are hidden in Windows using Folder options.

Hidden file extensions may cause a security threat because the file appearing as .doc or .txt may be .doc.exe or .txt.exe in actual and these exe file may be source of virus.

To enable file extensions for files in particular directory or your entire system you have to change default windows setting.

Open My Computer or any folder in your machine.

Go to Tools menu.

Select Folder Options.

Click on View Tab.

go to Hide extensions for known file types and uncheck the box

Apply settings.

After this file extension will be displayed in your system. To hide the extension check the box for Hide extensions for known file types