add extensions to file_managed entities

addext.php

<?php

use Drupal\Core\File\FileSystem;
use Drupal\file\Entity\File;

$container = \Drupal::getContainer();

$eq = $container->get('entity.query');
$fs = $container->get('file_system');

$type_map = [
  'image/gif' => '.gif',
  'image/jpeg' => '.jpg',
  'image/png' => '.png',
];

$files = File::loadMultiple($eq->get('file')->execute());

foreach ($files as $file) {
  if (!file_has_extension($file)) {
    $type = mime_content_type($fs->realpath($file->getFileUri()));

    if (isset($type_map[$type])) {
      $newname = $file->getFileUri() . $type_map[$type];
      file_move($file, $newname);
    }
  }
}

function file_has_extension(File $file) {
  $extension = pathinfo($file->getFilename())['extension'];
  return ($extension !== NULL);
}