bug #35173 [Mailer][MailchimpBridge] Fix missing attachments when sending via Mandrill API (vilius-g)

This PR was merged into the 4.3 branch.

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

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

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.

This also adds previously missing attachment names.

Commits
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7b1bbb6190 [Mailer][MailchimpBridge] Fix missing attachments when sending via Mandrill API

Author: fabpot

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;
             }
         }