View source for Feed Handling/Type Determination

= Background =

About a third of web feeds are served with an incorrect MIME type, e.g. text/html. In order to be able to correctly identify web feeds as feeds, parse and handle them correctly we need to resort to '''content sniffing'''. 

= Content Sniffing =

Through bug 324985, the ability was added to necko to register content sniffers using the net-content-sniffers category entry. A contract id to a component implementing nsIContentSniffer is added. When any file is transferred, the content sniffer is invoked with the raw data, with the method <code>getMIMETypeFromContent</code>. It must look at the data and see if it can determine the type, regardless of what the channel thinks the type is.

= Feeds and Content Sniffing =

In <code>browser/components/feeds/src/nsFeedSniffer.cpp</code> is an implementation of nsIContentSniffer which (depending on the Content-Encoding) decompresses the data and then runs some basic tests against it looking for feed content. If it determines the data is a web feed, it sets the content type to <code>application/vnd.mozilla.maybe.feed</code>, so that the document will be handled specially elsewhere in the system.