Cyrillic filename problem

I solve a problem with SPF and score is low now, but result is the same.

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--=_9c76bebf0af3799b0f7723617ab727b3
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

--=_9c76bebf0af3799b0f7723617ab727b3
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename="1"

Maybe the problem in “inline”, not “attachment”. Can it be?
Content-Disposition: inline; filename="1"

Hello all. I found out the solution.

Short answer.
Put the following line in the beginning of functions.php of your theme:
setlocale(LC_CTYPE, 'ru_RU.utf8');

Long answer.
1. wp-content/plugins/contact-form-7/modules/file.php line 162
Here is the chain of functions that format the name of file

$filename = $file['name'];
$filename = wpcf7_canonicalize( $filename );
$filename = sanitize_file_name( $filename );
$filename = wpcf7_antiscript_file_name( $filename );
$filename = wp_unique_filename( $uploads_dir, $filename );

Firstly, wpcf7_canonicalize() returns “??????????” instead of filename.
Secondly, sanitize_file_name() returns “1”.
Here we are. Obviously, promblem is in wpcf7_canonicalize() function:

function wpcf7_canonicalize( $text ) {
	if ( function_exists( 'mb_convert_kana' )
	&& 'UTF-8' == get_option( 'blog_charset' ) ) {
		$text = mb_convert_kana( $text, 'asKV', 'UTF-8' );
	}

	$text = strtolower( $text );
	$text = trim( $text );
	return $text;
}

Specifically, the problem is the strtolower() function, it returns “????????”, because locale LC_CTYPE is “ru_RU.CP1251” (in my case). It can be checked with setlocale(LC_CTYPE, 0); So, setting UT8-8 solves the problem:
setlocale(LC_CTYPE, 'ru_RU.utf8');

Thank you very much, Alex! ?? Great!