MimeType.
<script src="lib/WMMimeType.js"></script>
<script>
console.log( WMMimeType.getMimeType("swf") ); // "application/x-shockwave-flash"
</script>
importScripts("lib/WMMimeType.js");
console.log( WMMimeType.getMimeType("m4a") ); // "audio/x-m4a"
var WMMimeType = require("lib/WMMimeType.js");
console.log( WMMimeType.getMimeType("json") ); // "application/json"