A filename extension is a suffix to the filename applied to indicate the file format of its contents. In Windows, usually, the file extension contains 3 letters (e.g. AVI, JPG, DOC, XML, PDF) or 4 letters (e.g. DOCX, XLSX).
By default, Windows hides file name extensions to make file names easier to read, but you can choose to make extensions visible. If you need change a file extension name in Windows, you need to first make the extension names visible:
Please open Windows Explorer, go to menu Tools, Folder Options.
Click on the View tab and if the option "Hide Extensions for Known File Types" is active, de-select it.
Then the extension name of the files will be shown in Windows Explorer and you may proceed to rename the file as you need. Right click on the filename you need to rename, then choose "Rename". Press "End" button to move to the end of the filename and change the extension name, and Press "Enter" to complete the extension name change. Windows will prompt you that changing the file name extension might cause the file to stop working properly. If you are certain that the extension you typed is one that the program you are using will understand, click Yes to confirm the change.
The following characters cannot be used in a file name (including extension name):
You cannot use any of the following characters in a file name: \ / ? : * " > < |