magento2/app/code/Magento/Backup/Controller/Adminhtml/Index/Download.php

<?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Backup\Controller\Adminhtml\Index;

use Magento\Framework\App\Action\HttpGetActionInterface;
use Magento\Framework\App\Filesystem\DirectoryList;

class Download extends \Magento\Backup\Controller\Adminhtml\Index implements HttpGetActionInterface
{
    /**
     * @var \Magento\Framework\Controller\Result\RawFactory
     */
    protected $resultRawFactory;

    /**
     * @param \Magento\Backend\App\Action\Context $context
     * @param \Magento\Framework\Registry $coreRegistry
     * @param \Magento\Framework\Backup\Factory $backupFactory
     * @param \Magento\Framework\App\Response\Http\FileFactory $fileFactory
     * @param \Magento\Backup\Model\BackupFactory $backupModelFactory
     * @param \Magento\Framework\App\MaintenanceMode $maintenanceMode
     * @param \Magento\Framework\Controller\Result\RawFactory $resultRawFactory
     */
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Framework\Registry $coreRegistry,
        \Magento\Framework\Backup\Factory $backupFactory,
        \Magento\Framework\App\Response\Http\FileFactory $fileFactory,
        \Magento\Backup\Model\BackupFactory $backupModelFactory,
        \Magento\Framework\App\MaintenanceMode $maintenanceMode,
        \Magento\Framework\Controller\Result\RawFactory $resultRawFactory
    ) {
        parent::__construct(
            $context,
            $coreRegistry,
            $backupFactory,
            $fileFactory,
            $backupModelFactory,
            $maintenanceMode
        );
        $this->resultRawFactory = $resultRawFactory;
    }

    /**
     * Download backup action
     *
     * @return void|\Magento\Backend\App\Action
     */
    public function execute()
    {
        /* @var $backup \Magento\Backup\Model\Backup */
        $backup = $this->_backupModelFactory->create(
            $this->getRequest()->getParam('time'),
            $this->getRequest()->getParam('type')
        );

        if (!$backup->getTime() || !$backup->exists()) {
            /** @var \Magento\Backend\Model\View\Result\Redirect $resultRedirect */
            $resultRedirect = $this->resultRedirectFactory->create();
            $resultRedirect->setPath('backup/*');
            return $resultRedirect;
        }

        $fileName = $this->_objectManager->get(\Magento\Backup\Helper\Data::class)->generateBackupDownloadName($backup);

        return $this->_fileFactory->create(
            $fileName,
            ['type' => 'filename', 'value' => $backup->getPath() . DIRECTORY_SEPARATOR . $backup->getFileName()],
            DirectoryList::VAR_DIR,
            'application/octet-stream',
            $backup->getSize()
        );
    }
}