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.