If you have a locally installed TWX version, you can get the current upload widgets by going to the ThingWorx folder that is deployed in your Tomcat webapps folder. You should find it at this path : \apache-tomcat-8.0.18\webapps\Thingworx\Common\thingworx\widgets\fileupload.
If you do not have access to a locally installed TWX, you can still inspect the file upload widget in a Mashup with the Developer Tools from your browser and see the ide and runtime js files in the Combined. js files.
Hope this helps,