One of the most reliable ways for software to distinguish different kinds of files on the Internet is through a media type, an identifier that's part of the Multipurpose Internet Mail Extensions standard.
Web servers return a Content-Type header that identifies the kind of file being returned, such as "text/html" for an HTML page, "image/gif" for a GIF graphics file, and "application/atom+xml" for Atom syndicated feeds.
RSS documents lack an an official media type.
If a media type was defined for RSS, when a user opened an RSS feed in a web browser, the browser could open the document with the user's preferred software -- just as browsers crank up an MP3 player when a link to an MP3 is clicked.
Media types must be requested from the Internet Assigned Numbers Authority with a formal RFC that chooses the desired type, explains the nature of the content, and describes why the type is needed.
The advisory board is teaming up with the RSS-DEV Working Group, the developers of RSS 1.0, on a shared request for a media type.
Jon Hanna, Bill Kearney, Greg Smith and I have prepared an application to request "application/rss+xml" as the official media type for RSS documents.
With MIME RSS will not bee Really Simple Syndication :( atleast for PHP developers