This guide will teach you how to hide / show file extensions such as .png .jpg .app etc by default on a Mac.

1. When on Desktop or in a finder window, click on 'Finder' in menu bar and select Preferences.

<img src="http://modmyi.com/images/sayam/500,000_Contest/Picture-16.png"></img>

<img src="http://modmyi.com/images/sayam/500,000_Contest/Picture-17.png"></img>

2. It will show the 'General' tab by default, select the 'Advanced' tab.

<img src="http://modmyi.com/images/sayam/500,000_Contest/Picture-18.png"></img>

3. Select / Deselect 'Show all file extensions' if you want all extensions to be visible / not. Also, to avoid the warning when changing extensions, deselect 'Show warning before changing an extensions'.

<img src="http://modmyi.com/images/sayam/500,000_Contest/Picture-19.png"></img>

Hope this helps