Although the HTTP request has content-type: multipart/form-data, the actual form data also needs to have its content-type identified, as well as filename in some cases:

POST /my/server/path HTTP/1.1
....
content-type: multipart/form-data; boundary=--------------------------794262681322510475281872
...
Transfer-Encoding: chunked

2fe5b
----------------------------794262681322510475281872
Content-Disposition: form-data; name="image"; filename="my-image.jpg"
Content-Type: image/jpeg

...a bunch of binary data...

The form-data package infers filename and content-type from fs.ReadStream because that information is available in the file that you've read to a stream. With a raw Buffer or Blob it's just the raw data and form-data cannot infer content type, so you need to set it explicitly:

formData.append('image', buffer, {filename: 'image.jpg', contentType: 'image/jpg'});

You need to adapt that to your server's API spec, but that's the general idea. See here for further detail.