template_preprocess_file_link will not work with a stdClass object, though it tries to

The function template_preprocess_file_link() in core/modules/file/file.module starts out like this:

function template_preprocess_file_link(&$variables) {
  $file = $variables['file'];
  $options = [];

  $file_entity = ($file instanceof File) ? $file : File::load($file->fid);
  // @todo Wrap in file_url_transform_relative(). This is currently
  // impossible. As a work-around, we currently add the 'url.site' cache context
  // to ensure different file URLs are generated for different sites in a
  // multisite setup, including HTTP and HTTPS versions of the same site.
  // Fix in https://www.drupal.org/node/2646744.
  $url = $file_entity->createFileUrl(FALSE);
  $variables['#cache']['contexts'][] = 'url.site';

  $mime_type = $file->getMimeType();
  // Set options as per anchor format described at
  // http://microformats.org/wiki/file-format-examples
  $options['attributes']['type'] = $mime_type . '; length=' . $file->getSize();

Note how it tries to be backward-compatible with the passing of a stdClass object in $variables['file'] by calling File::load() to load a File object. However, these lines will cause an exception:

  $mime_type = $file->getMimeType();
  $options['attributes']['type'] = $mime_type . '; length=' . $file->getSize();

because they are using $file, not $file_entity.

These lines should either be fixed to use $file_entity, or support for passing a stdClass object should be completely removed.