To change the output file extension for HTML or XHTML:
Include the leading dot. The default value is.htm.
Also specify the following, for interfile links:
The extension you supply for FileSuffix must match the extension for the output files in your project. See §19.6.2 Specifying a file extension for interfile links.
For DITA or DocBook output, see:
§15.2.3 Specifying DITA output options
§17.2.3.1 Changing the DocBook output file extension