The association between mime types and file extensions is managed by the Indexers in Jigsaw. For example, the html extension is associated to a FileResource with an HTTPFrame, the content-type of this HTTPFrame is text/html.
So, if you want to add a new extension/mime-type association, you just have to add the new extension in the default indexer under the "extensions" node.
Example: You want to add the xml extension. Add a FileResource associated to an HTTPFrame to the "extensions" node of the "default" indexer. Set the HTTPFrame content-type to text/xml. Now files with xml extension (test.xml for example) will be indexed like that.
For more details on indexers management, read the JigAdmin 2.0 documentation or the JigAdmin 1.0 documentation about indexers.