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use Bitrix\Crm\Service\Container;

/** @var int $entityTypeId */
/** @var int $itemId */
/** @var int[] $sourceFileIds */
/** @var string $ufCode */

$container = Container::getInstance();
$factory = $container->getFactory($entityTypeId);
$item = $factory?->getItem($itemId);

if (!$factory || !$item)
{
    throw new \RuntimeException('CRM item not found');
}

$field = $factory->getFieldsCollection()->getField($ufCode);
if (!$field || !$field->isFileUserField())
{
    throw new \RuntimeException('CRM file field not found');
}

$fileUploader = $container->getFileUploader();
$resultFileIds = [];

foreach ($sourceFileIds as $sourceFileId)
{
    $fileArray = \CFile::MakeFileArray((int)$sourceFileId);
    if (!is_array($fileArray) || $fileArray === [])
    {
        continue;
    }

    $checkResult = $fileUploader->checkFile($field, $fileArray);
    if (!$checkResult->isSuccess())
    {
        continue;
    }

    $savedFileId = $fileUploader->saveFilePersistently($field, $fileArray);
    if ($savedFileId > 0)
    {
        $resultFileIds[] = $savedFileId;
    }
}

$item->set($ufCode, $field->isMultiple() ? $resultFileIds : ($resultFileIds[0] ?? null));

$operationResult = $factory->getUpdateOperation($item)->launch();
if (!$operationResult->isSuccess())
{
    throw new \RuntimeException(
        implode('; ', array_map(static fn($e) => $e->getMessage(), $operationResult->getErrors()))
    );
}