I wanted to publish my blog with something other than the .html extension and after some experimentation and hunting around I found this:Blogger Forum – Forum
The way to do it is to change the main page extension (from index.html to whatever, e.g. index.aspx) and do the same with the archive page. Then you need to go into each post individually and re-publish it. It’s a bit of a pain, but does work.
A handy tip: download all of your pages (in html format) first and then re-upload them afterwards so that all inbound links actually go somewhere.