gitlab-org--gitlab-foss/app/assets/javascripts/static_site_editor/services/image_service.js

export const getBinary = file => {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result.split(',')[1]);
    reader.onerror = error => reject(error);
  });
};