Some user feedback on this point:
When I try to add a little extra feature such as images as background for the website I’m working on, I realized I can’t upload any file that’s not in data format(csv, json)
It’s true that at the moment, only text files are supported in VizHub.
A generic file upload flow would be a great new feature to add. Images are one example, but other file types may be useful as well such as audio or video files.