Set the default HTML file extension
You can set the default file extension of new HTML documents created in Microsoft Expression Web. This setting affects all methods of creating new HTML documents, except for the New From Existing Page command and pages generated for the website templates.
To set the default file extension of HTML documents
On the Tools menu, click Page Editor Options.
In the Page Editor Options dialog box, click the Authoring tab.
Under Default Document and Extension, under Default HTML File Extension, select the file extension you want.
Click OK.