Changes to HTTP Responder for static type examples

[indigo-jay]

The method to return a static file in this plugin seems to have generated some confusion. Suggestions:

• Update it with clear language that there are 3 ways to deliver static content:
  • Automatically via the /Resources/ directory
  • Semi-automatically by returning indigo.utils.return_static_file (handle_static_file_request shows this by delivering a custom 404 html file)
  • By streaming returning the contents of the file yourself (handle_static_file_request shows this)
• Change the indigo.utils.FILE_EXTENSION_MIME_MAP lookup to use mimetypes.guess_type instead since that is now built-in (at least with Python 3.10).