You can’t really rely on those last few characters in a URL. For example, I can have all of my pages end in ".do", but have the web server software treat them as ".php". The obvious ones you mention are for pages where the developer doesn’t care if the reader (you) know what kind of page it is.

A lot of web developers use "security by obscurity". They’ll make up file extensions that attempt to "hide" the real extension such as php, asp, etc. That deters would-be hackers since it’s a little harder to exploit websites based on the technology they use for their pages.