nextcloud/apps/files_external/ajax/addRootCertificate.php

<?php

OCP\JSON::checkAppEnabled('files_external');

if ( !($filename = $_FILES['rootcert_import']['name']) ) {
	header("Location: settings/personal.php");
	exit;
}

$fh = fopen($_FILES['rootcert_import']['tmp_name'], 'r');
$data = fread($fh, filesize($_FILES['rootcert_import']['tmp_name']));
fclose($fh);
$filename = $_FILES['rootcert_import']['name'];

$view = new \OC_FilesystemView('/'.\OCP\User::getUser().'/files_external/uploads');
if (!$view->file_exists('')) $view->mkdir('');

$isValid = openssl_pkey_get_public($data);

//maybe it was just the wrong file format, try to convert it...
if ($isValid == false) {
	$data = chunk_split(base64_encode($data), 64, "\n");
	$data = "-----BEGIN CERTIFICATE-----\n".$data."-----END CERTIFICATE-----\n";
	$isValid = openssl_pkey_get_public($data);
}

// add the certificate if it could be verified
if ( $isValid ) {
	$view->file_put_contents($filename, $data);
	OC_Mount_Config::createCertificateBundle();
} else {
	OCP\Util::writeLog("files_external", "Couldn't import SSL root certificate ($filename), allowed formats: PEM and DER", OCP\Util::WARN);
}

header("Location: settings/personal.php");
exit;