Error: Call to undefined method Drupal\Core\ProxyClass\File\MimeType\MimeTypeGuesser::guess() in Drupal\mailchimp_transactional\Plugi n\Mail\Mail->getAttachmentStruct() (line 280 of modules/contrib/mailchimp_transactional/src/Plugin/Mail/Mail.php

The minimum supported core version is 9.0.0 and the guessMimeType method was added in 9.1.0, so we'll need to add a conditional:

$guesser = \Drupal::service('file.mime_type.guesser');
if ($guesser instanceof \Symfony\Component\Mime\MimeTypeGuesserInterface) {
  $mime_type = $guesser->guessMimeType($path);
}
else {
  $mime_type = $guesser->guess($path);
}

My comment hasn't been addressed yet, so setting back to Needs work.

Adding the updated patch based on the recent DieterHolvoet's comment above and opened merge request. This can go the version 1.1.x as well.

Regards,
Ramprassad

Merge request #21 is the valid one.
Merge request #22 was mistakenly raised with the intent to merge to 1.1.x but the branch was not available.

DieterHolvoet → closed merge request !22

Automatically closed - issue fixed for 2 weeks with no activity.