MC-15295: Fix integration, attachment

Author: Stepa4man

dev/tests/integration/testsuite/Magento/Framework/Mail/EmailMessageTest.php

@@ -210,6 +210,7 @@ public function testEmailMessageWithAttachment(): void
                 'content' => $this->getXmlContent(),
                 'disposition' => MimeInterface::DISPOSITION_ATTACHMENT,
                 'fileName' => self::ATTACHMENT_FILE_NAME,
+                'encoding' => MimeInterface::ENCODING_8BIT,
                 'type' => self::XML_TYPE
             ]
         );