Commit an image to a gist

Can we replicate this in javascript?
https://gist.github.com/mroderick/1afdd71aa69f6b29601d335751a1a9be

When I try to write it with gist.update(...), I get a bunch of binary garbage uploaded.

I can however commit images to github with `{encode: false}`. Can a similar parameter be passed to gist.update?