function getFile(url) {
  const groupPattern = /~\d+\/nth\/\d+\//;
  const uploaded = url.startsWith(CDN_BASE_URL) && !groupPattern.test(url);
  return uploadcare.fileFrom(uploaded ? 'uploaded' : 'url', url);
}
Convert a single url to an Uploadcare file object wrapped in a promise-like
object. Group urls that get passed here were not a part of a complete and
untouched group, so they'll be uploaded as new images (only way to do it).