问为什么下载的图像会被损坏？

EN

    $sql = "SELECT * FROM files WHERE  file_id= ? ";
    $params = array((int)$_POST["btn_save_file"]);
    $table = sqlsrv_query( $conn, $sql, $params);
    $row = sqlsrv_fetch_array( $table, SQLSRV_FETCH_ASSOC);
    $message=sqlsrv_errors();
    $content =$row["file_data"];
    $temp = tmpfile();
    fwrite($temp,$content);
    $name="img".rand(1,1000).".jpg";
    $a=fstat ($temp);
    header('Content-type: image/jpg');
    header('Content-Length: '.$a["size"]);
    header("Content-Disposition: attachment; filename=".$name);
    echo $content;
    fclose($temp);