[Mailer][MailchimpBridge] Fix missing attachments when sending via Mandrill API

Previous code tries to pass attachments to API, but uses incorrect structure and as a result all attachments are missing when the email is sent.

Author: vilius-g

Mailchimp/Http/Api/MandrillTransport.php

@@ -79,10 +79,14 @@ private function getPayload(Email $email, SmtpEnvelope $envelope): array
                 'type' => $headers->get('Content-Type')->getBody(),
             ];
 
+            if ($name = $headers->getHeaderParameter('Content-Disposition', 'name')) {
+                $att['name'] = $name;
+            }
+
             if ('inline' === $disposition) {
-                $payload['images'][] = $att;
+                $payload['message']['images'][] = $att;
             } else {
-                $payload['attachments'][] = $att;
+                $payload['message']['attachments'][] = $att;
             }
         }