This Gist shows an otherwise (AFAICT) undocumented way to upload images using the Imgur API with type=file.
I did not want to base64 encode (type=base64) the file contents because my use case was within a Cordova app where resources may limited.
The file is taken from the device file system, so no URL is accessible for file=url.
I was unable to find any guidance or examples in online documentation using this method in Javascript (or any language, AFAIR). I emailed(!) the suggested support contact laying out the problem and stressing that I had not found examples in the documentation. The reply came a month later, pointing me to the very docs in which I had told them I found no satisfactory answer or example! Gee, thanks.
It wasn't too hard, playing with cURL, to get a working call from command line: