rules.php

<?php
public function rules()
{
    return [
        "titulo" => "required",
        "contenido" => "required",
        "fotos" => "sometimes|required|array",
        "fotos.*" => [
            new RequiredIf($this->input("fotos")),
            "image",
            "mimes:jpeg,png",
            "max:3000",
        ]

    ];
}