From Joomla! Documentation

Adding support for new document types

New document types are added by creating a new sub-directory under the /libraries/joomla/document/ directory with the same name as the type. For example, to add a document type called "mytype", you would create the directory /libraries/joomla/document/mytype. In this directory you must then create a file called mytype.php which will contain the class definition for JDocumentMytype which extends JDocument. Look at the code for existing document types to see what needs to be done.

Chris Davenport 12:35, 17 April 2011 (CDT) Edit comment