If you want to implement “add file attachment in the email” functionality in Magento 2, then please follow the below step.

Step 1) Create Vendor/Module/etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
  <preference for="MagentoFrameworkMailTemplateTransportBuilder" type="VendorModuleModelMailTransportBuilder" />
</config>

Step 2) Please create the email template in Vendor/Module/view/frontend/email/email_template.html
then after calling this file to Vendor/Module/etc/email_templates.xml

Step 3) CreateTransportBuilder and extend with MagentoFrameworkMailTemplateTransportBuilder

Create Vendor/Module/Model/Mail/TransportBuilder.php

<?php
namespace VendorModuleModelMail;

use Zend_Mime;
class TransportBuilder extends MagentoFrameworkMailTemplateTransportBuilder
{
    public function addAttachment(
        $body,
        $mimeType    = Zend_Mime::TYPE_OCTETSTREAM,
        $disposition = Zend_Mime::DISPOSITION_ATTACHMENT,
        $encoding    = Zend_Mime::ENCODING_BASE64,
        $filename    = null
    ) {
        $this->message->createAttachment($body, $mimeType, $disposition, $encoding, $filename);
        return $this;
    }
}

Step 4) Send mail using controller in execute method
Create Vendor/Module/Controller/Index/Index.php

<?php
namespace VendorModuleControllerIndex;

use MagentoFrameworkAppActionContext;
use MagentoFrameworkUrlFactory;
use MagentoFrameworkAppFilesystemDirectoryList;
use MagentoFrameworkTranslateInlineStateInterface;
use VendorModuleModelMailTransportBuilder;
use MagentoFrameworkAppConfigScopeConfigInterface;
use MagentoStoreModelScopeInterface;
use Zend_Mime;


class Index extends MagentoCustomerControllerAbstractAccount {
    
    public function __construct(Context $context, 
        MagentoFrameworkFilesystem $fileSystem,
        StateInterface $inlineTranslation,
        TransportBuilder $transportBuilder,  
        ScopeConfigInterface $scopeConfig,
        UrlFactory $urlFactory, 
        MagentoFrameworkMessageManagerInterface $messageManager
    ) {
        $this->_filesystem = $fileSystem;
        $this->urlModel = $urlFactory->create ();
        $this->inlineTranslation = $inlineTranslation;
        $this->_transportBuilder = $transportBuilder;
        $this->scopeConfig = $scopeConfig;
        $this->messageManager = $messageManager;
        parent::__construct ( $context );
    }
    public function execute() {
        $objectManager = MagentoFrameworkAppObjectManager::getInstance();
        $storeurl = $objectManager->get( 'MagentoStoreModelStoreManagerInterface' )->getStore()->getBaseUrl();
        $result = array();
        $resultRedirect = $this->resultRedirectFactory->create ();
        if($_FILES['fileattach']['name']){
            try {
                $uploader = $objectManager->create('MagentoMediaStorageModelFileUploader',['fileId' => 'fileattach']);
                $uploader->setAllowedExtensions(['docx']);
                $uploader->setAllowRenameFiles(true);
                $uploader->setFilesDispersion(true);
                // get media directory
                $mediaDirectory = $this->_filesystem->getDirectoryRead(DirectoryList::MEDIA);
                // save the image to media directory
                
                $result = $uploader->save($mediaDirectory->getAbsolutePath()."fileupload");
                $email = $this->scopeConfig->getValue('trans_email/ident_general/email',ScopeInterface::SCOPE_STORE);
                $name  = $this->scopeConfig->getValue('trans_email/ident_general/name',ScopeInterface::SCOPE_STORE);
                $pdfFile = $result['path'].$result['file'];
                $emailTemplateVariables['message'] = 'This is a test message.';
                $this->inlineTranslation->suspend();
                $this->_transportBuilder->setTemplateIdentifier('email_template') // Add your template name
                  ->setTemplateOptions(
                      [
                          'area' => MagentoFrameworkAppArea::AREA_FRONTEND,
                          'store' =>  $objectManager->get( 'MagentoStoreModelStoreManagerInterface' )->getStore()->getId(),
                      ]
                  )
                  ->setTemplateVars($emailTemplateVariables)
                  ->setFrom([
                      'name' => 'From Name',
                      'email' => "test1@gmail.com",
                  ])
                  ->addTo("test123@gmail.com", "To Name")
                  ->addAttachment(file_get_contents($pdfFile),$result['type'],Zend_Mime::DISPOSITION_ATTACHMENT,Zend_Mime::ENCODING_BASE64,$result['name']); //Attachment goes here.
              try {
                  $transport = $this->_transportBuilder->getTransport();
                  $transport->sendMessage();
                  $this->inlineTranslation->resume();
              } catch (Exception $e) {
                  echo $e->getMessage(); die;
              }
            } catch (Exception $e) {
                Zend_Debug::dump($e->getMessage());
            }
        }else{
            $url = $this->urlModel->getUrl ( '*/*/create', [ 
                    '_secure' => true 
            ] );
            $resultRedirect->setUrl ( $this->_redirect->error ( $url ) );
            $this->messageManager->addError(__('Please upload the document'));
            return $resultRedirect;
        }
      }
    }