chromium/third_party/blink/web_tests/http/tests/download/resources/literal-koi8-r.php

<?php
header("Content-Disposition: attachment; filename=SU\xf3\xf3\xe5SS.txt");
header("Content-Type: text/plain; charset=windows-1251"); // Content-Type charset does not affect header charset.
?>

Test file content.