diff --git a/src/Controller/Mail/MailAttachmentDownloadController.php b/src/Controller/Mail/MailAttachmentDownloadController.php
new file mode 100644
index 0000000..f2be423
--- /dev/null
+++ b/src/Controller/Mail/MailAttachmentDownloadController.php
@@ -0,0 +1,93 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controller\Mail;
+
+use App\Mail\Exception\MailProviderException;
+use App\Mail\MailProviderInterface;
+use App\Repository\MailMessageRepository;
+use App\Security\MailAccessChecker;
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+use Symfony\Component\Routing\Attribute\Route;
+use Symfony\Component\Security\Http\Attribute\IsGranted;
+
+#[Route('/api/mail/attachments/{downloadId}', name: 'mail_attachment_download', methods: ['GET'], priority: 1)]
+#[IsGranted('IS_AUTHENTICATED_FULLY')]
+class MailAttachmentDownloadController extends AbstractController
+{
+    public function __construct(
+        private readonly MailMessageRepository $messageRepository,
+        private readonly MailProviderInterface $mailProvider,
+        private readonly MailAccessChecker $accessChecker,
+    ) {}
+
+    public function __invoke(string $downloadId): Response
+    {
+        $this->accessChecker->ensureCanAccessMail($this->getUser());
+
+        $decoded = base64_decode(strtr($downloadId, '-_', '+/'), true);
+        if (false === $decoded || !str_contains($decoded, ':')) {
+            throw new BadRequestHttpException('Invalid attachment ID format');
+        }
+
+        [$messageDbIdStr, $partNumber] = explode(':', $decoded, 2);
+        $messageDbId                   = (int) $messageDbIdStr;
+
+        $message = $this->messageRepository->find($messageDbId);
+        if (null === $message) {
+            throw new NotFoundHttpException('Message not found');
+        }
+
+        try {
+            $detail = $this->mailProvider->fetchMessage(
+                $message->getFolder()->getPath(),
+                $message->getUid()
+            );
+        } catch (MailProviderException) {
+            throw new NotFoundHttpException('Could not fetch message from IMAP server');
+        }
+
+        $targetAttachment = null;
+        foreach ($detail->attachments as $att) {
+            if ($att->partNumber === $partNumber) {
+                $targetAttachment = $att;
+
+                break;
+            }
+        }
+
+        if (null === $targetAttachment) {
+            throw new NotFoundHttpException(sprintf('Attachment part "%s" not found', $partNumber));
+        }
+
+        try {
+            $content = $this->mailProvider->fetchAttachment(
+                $message->getFolder()->getPath(),
+                $message->getUid(),
+                $partNumber
+            );
+        } catch (MailProviderException) {
+            throw new NotFoundHttpException('Could not fetch attachment content');
+        }
+
+        $filename = basename($targetAttachment->filename);
+        if ('' === $filename || '.' === $filename) {
+            $filename = 'attachment';
+        }
+
+        $response = new Response($content);
+        $response->headers->set('Content-Type', $targetAttachment->mimeType);
+        $response->headers->set(
+            'Content-Disposition',
+            sprintf('attachment; filename="%s"', addslashes($filename))
+        );
+        $response->headers->set('Content-Length', (string) strlen($content));
+        $response->headers->set('X-Content-Type-Options', 'nosniff');
+
+        return $response;
+    }
+}