The default setting for Mac OS is to not display a file’s extension. This tutorial will provide information on how to make it so you view the extensions for all files on your computer or for just an individual one. If you wish to hide the extension then you can check this box to have the Mac OS display this file’s. The Mac OS has now been configured to display the extension for this particular file. If you have any questions about this process please feel free to post them in our Mac OS Forum.
Source: https://www.bleepingcomputer.com/tutorials/show-file-extensions-on-a-mac/