foreach ($email->getAttachments() as $file) {
    $sendSmtpEmailAttachment = (new SendSmtpEmailAttachment())
        ->setName($file->getFilename())
        ->setContent(base64_encode($file->getContent()));

    $attachments[] = $sendSmtpEmailAttachment;
}

$sendSmtpEmail->setAttachment($attachments);

$attachments = [];
foreach ($email->getAttachments() as $file) {
    $attachments[] = [
        'name' => $file->getFilename(),
        'content' => base64_encode($file->getContent()),
    ];
}

$sendSmtpEmail->setAttachment($attachments);