This is a very simple page of HTML. I have saved two identical copies, one as webtest.html, the other as webtest.txt.
You will find that when fetching this page from a web server, MS Internet Explorer ignores the "Content-type:" specified by the server, and also pays no attention to the part of the URL that looks like a file extension. It just sees that it looks like HTML, and treats it as HTML. When following a file: URL, it does pay attention to the file extension.