How to build multipart/formdata with binary content in SM9 JavaScript

This would have easily been solved with the FormData() object:  https://javascript.info/formdata

Anyone managed to fix this in SM9.72 version?  I know it's easy to use base64Encoding, but tools like Jira doesn't like this encoding.  Currently we have a workaround letting 'curl' handle the attachment transfer but it is as I say just a workaround.