When migrating to the latest Sitefinity from the old version 3.7, by default the original .aspx page extension remains intact. It is stored in the Extension property of the PageNode.
You can remove (or change) this extension with this simple code snippet:
using (var api = App.WorkWith())
{
var pages = api.Pages().LocatedIn(PageLocation.Frontend).ThatArePublished()
.ForEach(page =>
{
page.Extension = null;
}).SaveChanges();
}