New installations of Windows default to hiding the file extension when file names appear in Explorer windows and Open/Save dialog boxes in applications. However, this important element of the file name can easily be made visible, and it is one of the first things a user should do after installing Windows. See extension
Windows 8.1 and 10
1. Launch Explorer and select View.
2. Check File name extensions.
Windows XP and 7
1. Launch Explorer and select Tools/Folder Options/View
2. Uncheck Hide extensions for known file types.