BookStack/app/Providers/CustomValidationServiceProvider.php

<?php

namespace BookStack\Providers;

use Illuminate\Support\Facades\Validator;
use Illuminate\Support\ServiceProvider;

class CustomValidationServiceProvider extends ServiceProvider
{

    /**
     * Register our custom validation rules when the application boots.
     */
    public function boot(): void
    {
        Validator::extend('image_extension', function ($attribute, $value, $parameters, $validator) {
            $validImageExtensions = ['png', 'jpg', 'jpeg', 'gif', 'webp'];
            return in_array(strtolower($value->getClientOriginalExtension()), $validImageExtensions);
        });

        Validator::extend('no_double_extension', function ($attribute, $value, $parameters, $validator) {
            $uploadName = $value->getClientOriginalName();
            return substr_count($uploadName, '.') < 2;
        });

        Validator::extend('safe_url', function ($attribute, $value, $parameters, $validator) {
            $cleanLinkName = strtolower(trim($value));
            $isJs = strpos($cleanLinkName, 'javascript:') === 0;
            $isData = strpos($cleanLinkName, 'data:') === 0;
            return !$isJs && !$isData;
        });
    }
}