The reason for this is, if you want to add your own extension, you need to specify a filename and a path to the file. This means that you need to specify the extension you want to use for your website.
The best way to find out what extension the file you’re working with is using the file name extension in the browser’s file name. For example if you are working on a website that is using the.js file extension, you need to go to the folder where you want to save your file, right-click on the.js file and select “Save File as…”. The extension will be the same as the file’s file name.