3398045 mailchimp deprecation error fix for sending attachments

Closes #3398045

src/Plugin/Mail/Mail.php

@@ -11,6 +11,7 @@ use Drupal\Core\Url;
 use Drupal\Core\Mail\MailFormatHelper;
 use Drupal\Component\Utility\Html;
 use Drupal\mailchimp_transactional\MailchimpTransactionalInterface;
+use Symfony\Component\Mime\MimeTypeGuesserInterface;
 
 /**
  * Allow Drupal mailsystem to use Mailchimp Transactional when sending emails.
@@ -277,7 +278,12 @@ class Mail implements MailInterface {
     $filename = basename($path);
     $file_buffer = file_get_contents($path);
     $file_buffer = chunk_split(base64_encode($file_buffer), 76, "\n");
-    $mime_type = $this->mimeTypeGuesser->guess($path);
+    if ($this->mimeTypeGuesser instanceof MimeTypeGuesserInterface) {
+      $mime_type = $this->mimeTypeGuesser->guessMimeType($path);
+    }
+    else {
+      $mime_type = $this->mimeTypeGuesser->guess($path);
+    }
     if (!$this->isValidContentType($mime_type)) {
       throw new \Exception($mime_type . ' is not a valid content type.');
     }