SesClientを利用して。sendEmailでは出来ないようなので。
$client = SesClient::factory([
                                 'key'     => 'aws key',
                                 'secret'  => 'aws secret',
                                 'version' => 'latest',
                                 'region'  => '', // us-east-1など
                             ]);

$to = "xxx@xxx";
$from = "xxx@xxx";

$subject = "テストメール件名です";
$body = "テストメール\n本文です";

$message = createRawMessage($to, $from, $subject, $body, ["attach.pdf", "attach.jpg"]);

$result = $client->sendRawEmail([
                                    'Source'       => $from,
                                    'Destinations' => [$to],
                                    'RawMessage'   => [
                                        'Data' => base64_encode($message),
                                    ],
                                ]);



function createRawMessage($to, $source, $subject, $body, $filepaths)
{

    $charset = 'ISO-2022-JP';
    $finfo = finfo_open(FILEINFO_MIME_TYPE);

    $boundaryStr = uniqid(rand()); //バウンダリ文字列
    $subject = str_replace("\r", "", mb_encode_mimeheader($subject));
    $body = mb_convert_encoding($body, $charset);

    $message = "To: " . $to . "\n";
    $message .= "From: " . $source . "\n";
    $message .= "Subject: " . $subject = str_replace("\r", "", $subject) . "\n";
    $message .= "MIME-Version: 1.0\n";
    $message .= 'Content-Type: multipart/mixed; boundary="' . $boundaryStr . '"';
    $message .= "\n\n";
    $message .= "--" . $boundaryStr . "\n";
    $message .= 'Content-Type: text/plain; charset=' . $charset;
    $message .= "\n";
    $message .= "Content-Transfer-Encoding: 7bit\n";
    $message .= "Content-Disposition: inline\n";
    $message .= "\n";
    $message .= $body;
    $message .= "\n";
    foreach ($filepaths as $filepath) {
        $message .= "\n";
        $path_parts = pathinfo($filepath);
        $filename = $path_parts['basename'];
        $mimetype = finfo_file($finfo, $filename);
        $message .= "--" . $boundaryStr . "\n";
        $message .= 'Content-Type: ' . $mimetype . '; name="' . $filename . '"';
        $message .= "\n";
        $message .= "Content-Transfer-Encoding: base64\n";
        $message .= 'Content-Disposition: attachment; filename="' . $filename . '"';
        $message .= "\n\n";
        $message .= base64_encode(file_get_contents($filepath));
        $message .= "\n";
    }
    return $message;

}
謝辞参考URL
https://www.codeproject.com/articles/786596/how-to-use-amazon-ses-to-send-email-from-php