Cara membuat upload file yang aman

Bagaimana cara membuat page upload file gambar yang aman ,

Disini saya mempunyai code

if(isset($_POST['tombol']))
{
    $temp = $_FILES['gambar']['tmp_name'];
    $name = rand(0,9999).$_FILES['gambar']['name'];
    $size = $_FILES['gambar']['size'];
    $type = $_FILES['gambar']['type'];
    $keterangan = $_POST['keterangan'];
    $folder = "files/";
    if ($size < 2048000 and ($type =='image/jpeg' or $type == 'image/png' or $type == 'image/jpg')) {
        move_uploaded_file($temp, $folder . $name);
        mysqli_query($koneksi, "insert into tb_gambar (gambar,keterangan,tipe_gambar,ukuran_gambar) values ('$name','$keterangan','$type','$size')");
        header('location:index.php');
    }else{
        echo "<b>Gagal Upload File</b>";
    }
}

Bagaimana agar page upload file tsb aman dari upload Shell dan lain lain,

Dan bagaimana cara agar maks size melebihi 2mb ,