diff --git a/src/Service/MailService.php b/src/Service/MailService.php
index 34e35ac..e7d8bd9 100644
--- a/src/Service/MailService.php
+++ b/src/Service/MailService.php
@@ -96,18 +96,19 @@ public function attachFiles(): false|Email
             return false;
         }
 
-        $mimeMessage = $this->message->getBody();
+        $body  = $this->message->getBody();
+        $parts = [];
 
-        //generate a new Part for each attachment
         foreach ($this->attachments as $key => $attachment) {
             if (! is_file($attachment)) {
                 continue;
             }
-            $basename     = is_string($key) ? $key : basename($attachment);
-            $attachedFile = new DataPart(fopen($attachment, 'r'), $basename);
-            $mimeMessage  = new MixedPart($mimeMessage, $attachedFile);
+            $basename = is_string($key) ? $key : basename($attachment);
+            $parts[]  = new DataPart(fopen($attachment, 'r'), $basename);
+        }
 
-            $this->message->setBody($mimeMessage);
+        if (count($parts) > 0) {
+            $this->message->setBody(new MixedPart($body, ...$parts));
         }
 
         return $this->message;
diff --git a/test/Service/MailServiceTest.php b/test/Service/MailServiceTest.php
index db8c13d..41eed41 100644
--- a/test/Service/MailServiceTest.php
+++ b/test/Service/MailServiceTest.php
@@ -19,6 +19,8 @@
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
 use Symfony\Component\Mailer\Transport\TransportInterface;
+use Symfony\Component\Mime\Part\DataPart;
+use Symfony\Component\Mime\Part\Multipart\MixedPart;
 use Symfony\Component\Mime\Part\TextPart;
 
 class MailServiceTest extends TestCase
@@ -153,4 +155,139 @@ public function testMailResultCreatedFromException(): void
         $this->assertSame($customException, $mailResult->getException());
         $this->assertSame('Custom exception test', $mailResult->getMessage());
     }
+
+    public function testAttachFilesReturnsFalseWhenNoAttachments(): void
+    {
+        $this->message->html('Test body');
+        $result = $this->mailService->attachFiles();
+        $this->assertFalse($result);
+    }
+
+    public function testAttachFilesCreatesFlatMixedPart(): void
+    {
+        $this->message->html('<p>Test</p>');
+
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf'
+        );
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testXlsAttachment.xls'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
+
+        $parts = $body->getParts();
+        // 1 TextPart (html) + 2 DataParts (attachments) = 3 parts at same level
+        $this->assertCount(3, $parts);
+        $this->assertInstanceOf(TextPart::class, $parts[0]);
+        $this->assertInstanceOf(DataPart::class, $parts[1]);
+        $this->assertInstanceOf(DataPart::class, $parts[2]);
+    }
+
+    public function testAttachFilesSingleAttachmentIsFlat(): void
+    {
+        $this->message->html('<p>Single attachment</p>');
+
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
+
+        $parts = $body->getParts();
+        $this->assertCount(2, $parts);
+        $this->assertInstanceOf(TextPart::class, $parts[0]);
+        $this->assertInstanceOf(DataPart::class, $parts[1]);
+    }
+
+    public function testAttachFilesSkipsNonExistentFiles(): void
+    {
+        $this->message->html('<p>Test</p>');
+
+        $this->mailService->addAttachment('/nonexistent/file.pdf');
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
+
+        $parts = $body->getParts();
+        // only 1 valid attachment + the html body
+        $this->assertCount(2, $parts);
+    }
+
+    public function testAttachFilesPreservesCustomFilename(): void
+    {
+        $this->message->html('<p>Test</p>');
+
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf',
+            'custom-ticket.pdf'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body  = $this->message->getBody();
+        $parts = $body->getParts();
+
+        $this->assertCount(2, $parts);
+        $attachment = $parts[1];
+        $this->assertInstanceOf(DataPart::class, $attachment);
+        $this->assertSame('custom-ticket.pdf', $attachment->getFilename());
+    }
+
+    public function testAttachFilesWithTextPartBody(): void
+    {
+        $textPart = new TextPart('<h1>HTML content</h1>', 'utf-8', 'html');
+        $this->mailService->setBody($textPart);
+
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf'
+        );
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testXlsAttachment.xls'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body = $this->message->getBody();
+        $this->assertInstanceOf(MixedPart::class, $body);
+
+        $parts = $body->getParts();
+        $this->assertCount(3, $parts);
+        $this->assertInstanceOf(TextPart::class, $parts[0]);
+        $this->assertInstanceOf(DataPart::class, $parts[1]);
+        $this->assertInstanceOf(DataPart::class, $parts[2]);
+    }
+
+    public function testAttachFilesNoNestedMixedParts(): void
+    {
+        $this->message->html('<p>Test nesting</p>');
+
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testPdfAttachment.pdf'
+        );
+        $this->mailService->addAttachment(
+            $this->fileSystem->url() . '/data/mail/attachments/testXlsAttachment.xls'
+        );
+
+        $this->mailService->attachFiles();
+
+        $body  = $this->message->getBody();
+        $parts = $body->getParts();
+
+        // None of the children should be a MixedPart (no nesting)
+        foreach ($parts as $part) {
+            $this->assertNotInstanceOf(MixedPart::class, $part);
+        }
+    }
 }