Attachment in handler

When using Postmark templates to send emails with attachments via the Postmark module, multiple patches are currently needed to enable both functionalities. The patch from #3054592 successfully adds support for sending emails using Postmark templates but does not support attachments.

Meanwhile, patches from #3281233 and #3463840 address sending attachments, but do not account for template usage. Unfortunately, when both template and attachment support are required, the patch from #3463840 cannot be applied due to conflicts with the changes introduced in #3054592.

To address this issue, I have created a combined patch that integrates the logic from the following three patches:

1. Sending with Postmark templates ( #3054592 ✨ Sending with postmark templates Needs review )
   
   https://www.drupal.org/files/issues/2024-09-24/3054592-support-for-postm... →
2. Messages with file attachments fail to send ( #3281233 🐛 Messages with file attachments fail to send Needs review )
   
   https://www.drupal.org/files/issues/2023-04-17/3281233-2.patch →
3. Allow attachments in handler ( #3463840 🐛 Attachment in handler Active )
   
   https://www.drupal.org/files/issues/2024-07-25/allow_attachments_handler... →

This unified patch allows sending Postmark template emails with attachments, resolving the compatibility issues between the individual patches.