Do you know that File Extensions are usually hidden on your MacOS. So, how to show file extensions in case you desire to do it.
Make it clearer by using the following information for reference:
The first step is to open Finder. Next, click on Preferences .
Afterwards, select Advanced within Finder Preferences. Now you can see line Show all filename extensions, check the box next to it. By doing that, you can see file extensions in both Finder and desktop.
If you just want to show extention for chosen file, click on right mouse, then choose Get Info from the context menu. Now broaden Name and Extension group. When you see option Hide Extension, uncheck it so that file extension will be shown.