I've added the following in my code to pull in (more) mime types from the node-mime module (https://github.com/bentomas/node-mime):

var mime = require("mime");
var lightnode = require('lightnode');

var server = new lightnode.FileServer(__dirname + '/static');

server.mimeTypes = mime.types;

Seems undocumented, maybe utilizing node-mime in the core lightnode code would be a good idea...