Update MailServiceTest.php

Fix php stan errors

Signed-off-by: APaikens <shamanis@gmail.com>

Author: APaikens

test/Service/MailServiceTest.php

@@ -236,7 +236,8 @@ public function testAttachFilesPreservesCustomFilename(): void
 
         $this->mailService->attachFiles();
 
-        $body  = $this->message->getBody();
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
         $parts = $body->getParts();
 
         $this->assertCount(2, $parts);
@@ -282,7 +283,8 @@ public function testAttachFilesNoNestedMixedParts(): void
 
         $this->mailService->attachFiles();
 
-        $body  = $this->message->getBody();
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
         $parts = $body->getParts();
 
         // None of the children should be a MixedPart (no nesting)