To show file extension in sharepoint document library

1. Open up the .aspx file responsible for displaying the document list in Sharepoint Designer, in my case it is called AllItems.aspx

2. Select the WebPartPages:ListViewWebPart control. (It is the place where rows of document are listed)

3. Right-click on it, Convert to XSLT Data View

4. Go into the code and find the line that says:

<xsl:value-of select="ddwrt:UrlBaseName(string(@LinkFilename))" />

5. Add this to the end of the line:

.<xsl:value-of select="@File_x0020_Type" />

6. Save

Caution: after editing the file with sharepoint designer, you wouldn’t be able to edit it with anything else except maybe Visual Studio