RegenerateMediaIconsOnDeploy.php

<?php

/**
 * @file
 * mymodule module.
 */

use Drupal\Core\File\FileSystemInterface;

/**
 * Implements hook_preprocess_page().
 */
function mymodule_file_preprocess_page(&$vars) {
  if (\Drupal::routeMatch()->getRouteName() == 'entity.media.collection') {
    _mymodule_file_recreate_media_folder();
  };
}

/**
 * Helper function to recreate the media-icons public directory.
 *
 * @see media.install
 */
function _mymodule_file_recreate_media_folder() {
  $source = drupal_get_path('module', 'media') . '/images/icons';
  $destination = \Drupal::config('media.settings')->get('icon_base_uri');
  /** @var \Drupal\Core\File\FileSystemInterface $file_system */
  $file_system = \Drupal::service('file_system');
  $file_system->prepareDirectory($destination, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS);

  $files = $file_system->scanDirectory($source, '/.*\.(svg|png|jpg|jpeg|gif)$/');
  foreach ($files as $file) {
    // When reinstalling the media module we don't want to copy the icons when
    // they already exist. The icons could be replaced (by a contrib module or
    // manually), so we don't want to replace the existing files. Removing the
    // files when we uninstall could also be a problem if the files are
    // referenced somewhere else. Since showing an error that it was not
    // possible to copy the files is also confusing, we silently do nothing.
    if (!file_exists($destination . DIRECTORY_SEPARATOR . $file->filename)) {
      try {
        $file_system->copy($file->uri, $destination, FileSystemInterface::EXISTS_ERROR);
      }
      catch (FileException $e) {
        // Ignore and continue.
      }
    }
  }
}