Product::downloadAction

public function downloadAction(array $fields): ?EngineResponseBFile
{
	$productId = $fields['PRODUCT_ID'];
	$fieldName = $fields['FIELD_NAME'];
	$id = $fields['FILE_ID'];
	$file = [];

	$r = $this->exists($productId);
	if($r->isSuccess())
	{
		$iblockId = $this->get($productId)['IBLOCK_ID'];

		if($this->checkFieldsDownload(['NAME'=>$fieldName, 'IBLOCK_ID'=>$iblockId]) == true)
		{
			$files = [];
			$iBlock = CIBlock::GetArrayByID($iblockId);

			if ($productId > 0)
			{
				$element = CIBlockElement::GetList(
					array(),
					array(
						"CATALOG_ID" => $iBlock["ID"],
						"=ID" => $productId,
						"CHECK_PERMISSIONS" => "N",
					),
					false,
					false,
					array("ID", $fieldName)
				);
				while ($res = $element->GetNext())
				{
					if (isset($res[$fieldName]))
					{
						$files[] = $res[$fieldName];
					}
					elseif (isset($res[$fieldName."_VALUE"]))
					{
						if (is_array($res[$fieldName."_VALUE"]))
						{
							$files = array_merge($files, $res[$fieldName."_VALUE"]);
						}
						else
						{
							$files[] = $res[$fieldName."_VALUE"];
						}
					}
				}
			}

			if (!in_array($id, $files))
			{
				$r->addError(new Error('Product file wrong'));
			}
			else
			{
				$file = CFile::GetFileArray($id);
				if (is_array($file) == false)
				{
					$r->addError(new Error('Product is empty'));
				}
			}
		}
		else
		{
			$r->addError(new Error('Name file field is not available'));
		}
	}

	if($r->isSuccess())
	{
		return BitrixMainEngineResponseBFile::createByFileId($file['ID']);
	}
	else
	{
		$this->addErrors($r->getErrors());
		return null;
	}
}