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.