A config setting dictionary that pairs filename extension strings used in URLs as human controlled content-negotiation with the MIME types they represent. Add to this if you add to serializers.

The default values are as follows:

    'extension_types': {
        'txt': 'text/plain',
        'html': 'text/html',
        'json': 'application/vnd.tiddlyweb+json'},

As an example the tiddlywebwiki plugin adds a .wiki extension:

    'extension_types': {
        'wiki': 'text/x-tiddlywiki',
    },