uploading-files.js

postFile('http://example.com/api/v1/users', 'input[type="file"].avatar')
  .then(data => console.log(data))

function postFile(url, fileSelector) {
  const formData = new FormData()
  const fileField = document.querySelector(fileSelector)

  formData.append('username', 'abc123')
  formData.append('avatar', fileField.files[0])

  return fetch(url, {
    method: 'POST', // 'GET', 'PUT', 'DELETE', etc.
    body: formData  // Coordinate the body type with 'Content-Type'
  })
  .then(response => response.json())
  .catch(error => console.error(error))
}