How to Display File Extensions in a SharePoint Document Library - TechNet Articles - United States (English) - TechNet Wiki

There are 2 ways to show file extensions in a SharePoint document library

Method1

Convert the document library List View web part to a XSLT dataview
Open the document library with FrontPage,
Right click and convert to DataView.
Modify the XSLT to add select=”@File_x0020_Type”/>

This solution can be seen in context by clicking here.

Method2

Modify the display pattern in ONET.XML
Add
Name=”File_x0020_Type”/>
Before
Name=”Created_x0020_Date”>.

After making a change to ONET.XML you have to do IISRESET