fetch-success.js

const uploadInput = document.querySelector('input[type="file"]');
const awsFormData = JSON.parse(uploadEl.getAttribute('data-form-data'));
const formData = new FormData();

Object.keys(awsFormData).forEach((key)=>{
  formData.append(key, awsFormData[key]);
});

uploadInput.addEventListener('change', function(ev){
  ev.preventDefault();
  const file = ev.target.files[0];

  formData.append('Content-Type', file.type);
  formData.append('file', file);

  const config = {
    method: "POST",
    headers: new Headers({
        "Accept": "application/xml"
      }),
    body: formData,
  };

  return fetch(url, config)
    .then(response => response.text())
    .then((xml) => {
      // decode xml and handle response
    })
    .catch((e) => console.error.bind(console))
})

thanks, was struggling with Content-Type :D