<?php

final class DemoFile
{
    public function run(): JsonResponse
    {
        $thumbnails = [];
        $groups = [];
        $list = new DemoList();
        foreach ($list->getItems() as $item) {
            if ($item->hasGroup()) {
                $groups[] =['id' => $item->getName()];
            } else {
                $thumbnails[] = ['id' => $item->getName()];
            }
        }

        foreach ($groups as $group) {
            $thumbnails[] = $group;
        }
        
        return $this->adminJson($thumbnails);
    }
}

foreach ($groups as $group) {
     $thumbnails[] = $group;
}