Unfortunately, this is not supported out of the box so I don't believe that would be possible at this time. Using the "Manage Files" option (as outlined in Uploading Static Resources to a Widget) you may be able to upload certain static resources, but I don't believe this supports .js and other script files. If this is an important feature you'd like to see added in the future, I would encourage submitting an idea in the Ideas for Jive space to raise the attention to our Products Team. Otherwise, I would also recommend posting your question to the Jive Developers space to see how other community developers are workaround around this.
Let me know if you had any other questions on this.
Thanks Jeph Yang for the detailed information. That's fine, I was looking into this feature because I wanted to host some JS/CSS files on to the same server and then using in themes. So I would do upload from the jive itself and then grab the URL.