"This file type isn't supported.".
Is necesary to set up a wordpress plugin within the Joomla's platform to upload .js file, or upload in special path within the Joomla's platform?.
You need to access outdoors of the administration access and go straight to the files inside your template folder for that template you're using. You will find workarounds to complete a number of this from yoursite.com/administrator but you're restricted to web site you're using and also the functions they provide.