By default, document libraries do not show file extensions. You have to look at the icon to guess the kind of the document. However, multiple extensions can have the same icon. So, it could be useful to display the extension.

1. Open SharePoint Designer
2. Edit the document library view
3. Right click on the file name and select "Edit Formula"
4. Use the following XPath expression $thisNode/@FileLeafRef
5. Save changes

Voila 😃