In the Default Extension box, specify the file extension
you want for new HTML documents created in Dreamweaver.
For Windows, you can specify the following extensions:
.html, .htm, .shtml, .shtm, .stm, .tpl, .lasso, .xhtml.
For
Macintosh, you can specify the following extensions: .html, .htm,
.shtml, .shtm, .tpl, .lasso, .xhtml, .ssi.