How to add base64 data to FormData so that it's uploaded as binary

Question

I have a API source of image data in base64 format. I'm trying to upload it as normal binary upload.

From MDN I can see that my problem is, given the Blob is available, reduced to making binary Blob from base64:

var content = 'iVBORw0KGgoAAAANSUhEUgAABUUAAAI/...'; // the image
var blob = new Blob([content], { type: "image/png,base64"});

formData.append("image", blob);

How can I do this safely (=avoid wild unicode strings)?