If you have a custom view on document library where you need to show the each file name with extension. This is fairly simple and can be done this way.
Open your document library view in SharePoint designer and follow these steps
Open your web site the SharePoint designer
Go to you document library and open the yourviewname.aspx file in design view.
Select the WebPartPages:ListViewWebPart control
Right click on WebPartPages:ListViewWebPart control and select “Convert to XSLT data View”
Right click on the column where you want to show file extension and select “Insert formula”
In the right hand box delete the existing entry and insert @FileLeafRef
Click “Save” on the SharePoint designer ribbon.
Refresh the document library and now you will see the document name with extension in the new column.
Admin