MIME or Multipurpose Internet Mail Extensions

https://en.wikipedia.org/wiki/MIME

Started with SMTP but used by HTTP

servers insert a MIME header field at the beginning of any Web transmission.
Clients use the content type or media type header to select an appropriate viewer application for the type of data indicated.

Common MIME types:    https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types

List of MIME:   http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types

application/x-msdownload