Magento 2.3 – Custom Module Send Email Programmatically with Image Attachment

I've done this by creating below post controller for the custom form to send data with attachment in Magento 2.

<?php

namespace Vendor\Module\Controller\Index;

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

class Post extends \Magento\Framework\App\Action\Action {

    const XML_PATH_EMAIL_RECIPIENT = 'section/email/recipient_email';
    const XML_PATH_EMAIL_SENDER = 'section/email/sender_email_identity';
    const XML_PATH_EMAIL_TEMPLATE = 'section/email/email_template';


    /**
     * @var \Magento\Framework\Mail\Template\TransportBuilder
     */
    protected $_transportBuilder;

    /**
     * @var \Magento\Framework\Translate\Inline\StateInterface
     */
    protected $inlineTranslation;

    /**
     * @var \Magento\Framework\App\Config\ScopeConfigInterface
     */
    protected $scopeConfig;

    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $storeManager;

    /**
     * @var \Magento\Framework\Escaper
     */
    protected $_escaper;

    /**
     * @var \Magento\Framework\File\UploaderFactory
     */
    protected $uploaderFactory;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Framework\Mail\Template\TransportBuilder $transportBuilder
     * @param \Magento\Framework\Translate\Inline\StateInterface $inlineTranslation
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\MediaStorage\Model\File\UploaderFactory $uploaderFactory
     */
    public function __construct(
        Filesystem $filesystem,
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\Mail\Template\TransportBuilder $transportBuilder,
        \Magento\Framework\Translate\Inline\StateInterface $inlineTranslation,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\Escaper $escaper,
        \Magento\MediaStorage\Model\File\UploaderFactory $uploaderFactory
    ) {
        parent::__construct($context);
        $this->_mediaDirectory = $filesystem->getDirectoryWrite(DirectoryList::MEDIA);
        $this->_transportBuilder = $transportBuilder;
        $this->inlineTranslation = $inlineTranslation;
        $this->scopeConfig = $scopeConfig;
        $this->storeManager = $storeManager;
        $this->_escaper = $escaper;
        $this->uploaderFactory = $uploaderFactory;
    }

    /**
     * Post user question
     *
     * @return void
     * @throws \Exception
     */
    public function execute()
    {
        $post = $this->getRequest()->getPostValue();
        if (!$post) {
            $this->_redirect('*/*/');
            return;
        }

        $this->inlineTranslation->suspend();
        try {
            $postObject = new \Magento\Framework\DataObject();
            $postObject->setData($post);

            $error = false;
            if (!\Zend_Validate::is(trim($post['name']), 'NotEmpty')) {
                $error = true;
            }

            $photos = array();
            foreach ($_FILES['photo']['name'] as $key => $image) {

                if (empty($image)) {
                    continue;
                }

                $fileName = '';
                if (isset($_FILES['photo']['name'][$key]) && $_FILES['photo']['name'][$key] != '') {
                    try {
                        $target = $this->_mediaDirectory->getAbsolutePath('rga');
                        $fileName = $_FILES['photo']['name'][$key];
                        $fileExt = strtolower(substr(strrchr($fileName, "."), 1));

                        $fileNamewoe = rtrim($fileName, $fileExt);

                        $fileName = preg_replace('/[^A-Za-z0-9\-]/', '', $fileNamewoe) . time() . $key . '.' . $fileExt;
                        if (!in_array($fileExt, array('jpg', 'jpeg', 'png', 'gif'))) {
                            $this->messageManager->addError(__('Only jpg, jpeg, png and gif file types are allowed.'));
                            session_write_close();
                            $this->_redirect('*/*/');
                            return;
                        }
                        array_push($photos, $fileName);
                        $uploader = $this->uploaderFactory->create(['fileId' => 'photo['.$key.']']);
                        $uploader->setAllowedExtensions(['jpg', 'jpeg', 'png', 'gif']);
                        $uploader->setAllowRenameFiles(false);
                        $uploader->setFilesDispersion(false);
                        $uploader->save($target,$fileName);
                    } catch (Exception $e) {
                        $error = true;
                    }
                }
            }

            if ($error) {
                $this->messageManager->addError(__('Unable to submit your request. Please, try again later'));
                session_write_close();
                throw new \Exception();
            }

            $storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORE;
            $transport = $this->_transportBuilder
                ->setTemplateIdentifier($this->scopeConfig->getValue(self::XML_PATH_EMAIL_TEMPLATE, $storeScope)) // this code we have mentioned in the email_templates.xml
                ->setTemplateOptions(
                    [
                        'area' => \Magento\Framework\App\Area::AREA_FRONTEND, // this is using frontend area to get the template file
                        'store' => $this->storeManager->getStore()->getId(),
                    ]
                )
                ->setTemplateVars(['data' => $postObject])
                ->setFrom($this->scopeConfig->getValue(self::XML_PATH_EMAIL_SENDER, $storeScope))
                ->setReplyTo($post['EmailAddress'])
                ->addTo($this->scopeConfig->getValue(self::XML_PATH_EMAIL_RECIPIENT, $storeScope));

            /* add photos to attachment; */
            foreach($photos as $pic) {
                $attachmentFilePath = $this->_mediaDirectory->getAbsolutePath('rga').'/'. $pic;
                if(file_exists($attachmentFilePath)){
                    $transport->addAttachment(file_get_contents($attachmentFilePath,$pic));
                }
            }

            $transport = $transport->getTransport();           
            $transport->sendMessage(); 
            $this->inlineTranslation->resume();
            $this->messageManager->addSuccess(
                __('Your inquiry was submitted and will be responded to as soon as possible. Thank you for contacting us.')
            );
            $this->_redirect('*/*/');
            return;
        } catch (\Exception $e) {
            $this->inlineTranslation->resume();
            $this->messageManager->addError(
                __('Unable to submit your request. Please, try again later.')
            );
            $this->_redirect('*/*/');
            return;
        }
    }
}

Hope it helps!!!