Magento2 file download action

you can create your controller action by extending \Magento\Backend\App\Action for backend or \Magento\Framework\App\Action\Action for frontend.
and make it look like this:

<?php 
namespace Your\Namespace\Here;

class ClassName extends \Magento\Backend\App\Action 
{
    public function __construct(
        \Magento\Framework\Controller\Result\RawFactory $resultRawFactory,
        \Magento\Framework\App\Response\Http\FileFactory $fileFactory,
        \Magento\Backend\App\Action\Context $context
    ) {
        $this->resultRawFactory      = $resultRawFactory;
        $this->fileFactory           = $fileFactory;
        parent::__construct($context);
    }
    public function execute()
    {
        //do your custom stuff here
        $fileName = 'file name for download here';
        $this->fileFactory->create(
            $fileName,
            null, //content here. it can be null and set later 
            base dir of the file to download here
            'application/octet-stream', //content type here
            content lenght here...can be null
        );
        $resultRaw = $this->resultRawFactory->create();
        $resultRaw->setContents(contents of file here); //set content for download file here
        return $resultRaw;
    }
}

Also one can provide a path to a file you would like to be downloaded:

//Send file for download
//@see Magento\Framework\App\Response\Http\FileFactory::create()
return $this->_fileFactory->create(
    //File name you would like to download it by
    $filename,
    [
        'type'  => "filename", //type has to be "filename"
        'value' => "folder/{$filename}", // path will append to the
                                         // base dir
        'rm'    => true, // add this only if you would like the file to be
                         // deleted after being downloaded from server
    ],
    \Magento\Framework\App\Filesystem\DirectoryList::MEDIA
);

Based on the answer that Marius gave.

class Download extends \Magento\Framework\App\Action\Action
{
    protected $resultRawFactory;
    protected $fileFactory;

    public function __construct(
        \Magento\Framework\Controller\Result\RawFactory $resultRawFactory,
        \Magento\Framework\App\Response\Http\FileFactory $fileFactory,
        \Magento\Backend\App\Action\Context $context
    ) {
        $this->resultRawFactory      = $resultRawFactory;
        $this->fileFactory           = $fileFactory;
        parent::__construct($context);
    }
    public function execute()
    {
        try{
            $fileName = 'FileName'; // the name of the downloaded resource
            $this->fileFactory->create(
                $fileName,
                [
                    'type' => 'filename',
                    'value' => 'relative/path/to/file/from/basedir'
                ],
                DirectoryList::MEDIA , //basedir
                'application/octet-stream',
                '' // content length will be dynamically calculated
            );
        }catch (\Exception $exception){
            // Add your own failure logic here
            var_dump($exception->getMessage());
            exit;
        }
        $resultRaw = $this->resultRawFactory->create();
        return $resultRaw;
    }
}

Not having correct permissions (even though a read is needed here the Magento checks for write permissions) will result in a weird error. "The site is down or moved" or smth like that.

It's worth taking a sneak peak at the logic inside $fileFactory->create() as well.