diff --git a/src/Symfony/Component/Mailer/Bridge/MailerSend/Transport/MailerSendApiTransport.php b/src/Symfony/Component/Mailer/Bridge/MailerSend/Transport/MailerSendApiTransport.php
index 3531d94253f55..21c0fe4070d6d 100644
--- a/src/Symfony/Component/Mailer/Bridge/MailerSend/Transport/MailerSendApiTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/MailerSend/Transport/MailerSendApiTransport.php
@@ -147,7 +147,7 @@ private function prepareAttachments(Email $email): array
             $filename = $headers->getHeaderParameter('Content-Disposition', 'filename');
 
             $att = [
-                'content' => $attachment->bodyToString(),
+                'content' => base64_encode($attachment->getBody()),
                 'filename' => $filename,
             ];