Форум

?external_context=creatingElementFromCrm&fieldId=PROPERTY_144&defaultValue=CO_666

if (!empty($_FILES)) {

    // Здесь нужно проверить файлы на тип, размер и пр

    $driveInstance =  \Bitrix\Disk\Driver::getInstance();
    $storage = $driveInstance->getStorageByUserId($id);
    $folder = $storage->getFolderForUploadedFiles();

    $arFiles = [];

    foreach ($_FILES as $arFile) {

        $data = ['NAME' => $arFile['name'], 'CREATED_BY' => $id];
    
        if ($fileModel = $folder->uploadFile($arFile, $data, array(), true)) {
            $arFiles[] = 'n' . $fileModel->getId();
        }
    }
}

$arFields = [
    "AUTHOR_NAME" => $name,
    "AUTHOR_ID" => $id,
    "POST_MESSAGE" => $message,
    "FORUM_ID" => $forum,
    "TOPIC_ID" => $topic,
    "NEW_TOPIC" => $new ? "Y" : "N"
];

if (!empty($arFiles)) {
    $arFields["UF_FORUM_MESSAGE_DOC"] = $arFiles;
}

$messageId = \CForumMessage::Add($arFields);

global $APPLICATION;
if ($messageId <= 0 && $ex = $APPLICATION->GetException()) {
    die($ex);
}

$arFields = Array(
    "AUTHOR_NAME" => $name,
    "AUTHOR_ID" => $id,
    "POST_MESSAGE" => $message,
    "FORUM_ID" => $forum,
    "TOPIC_ID" => $topic,
    "NEW_TOPIC" => $new ? "Y" : "N"
);

$messageId = \CForumMessage::Add($arFields);

$driveInstance =  \Bitrix\Disk\Driver::getInstance();
$storage = $driveInstance->getStorageByUserId($id);

foreach ($_FILES as $arFile) {
    $folder = $storage->getFolderForUploadedFiles();
    $fileModel = $folder->uploadFile($arFile, ['NAME' => $arFile['name'], 'CREATED_BY' => $id, array(), true);

    ????
}