Get data from file for use with <image> src.

image-preview.js

const readFileData = (file) => {
  return new Promise(resolve => {
    if (!file instanceof File)
      return resolve(file);

    const reader = new FileReader();
    reader.addEventListener('load', function() {
      resolve(this.result);
    });

    reader.readAsDataURL(file);
  });
}