file-upload.js

function generateFormData() {
  // define upload field
  let file_input = document.getElementById('profile_picture');

  // define form data
  let data = new FormData();

  // Add regular text field
  data.append("name", "John Doe");

  // If any files added to file_input, add first
  if (file_input.files.length > 0) {
    data.append('profile_picture', file_input.files[0]);
  }

  return data;
}

// Now simply post the generated form data using ajax.