diff --git a/system/Email/Email.php b/system/Email/Email.php
index d18b40ff7cef..42835c3d404e 100644
--- a/system/Email/Email.php
+++ b/system/Email/Email.php
@@ -705,6 +705,7 @@ public function attach($file, $disposition = '', $newname = null, $mime = '')
     public function setAttachmentCID($filename)
     {
         foreach ($this->attachments as $i => $attachment) {
+            // For file path.
             if ($attachment['name'][0] === $filename) {
                 $this->attachments[$i]['multipart'] = 'related';
 
@@ -712,6 +713,15 @@ public function setAttachmentCID($filename)
 
                 return $this->attachments[$i]['cid'];
             }
+
+            // For buffer string.
+            if ($attachment['name'][1] === $filename) {
+                $this->attachments[$i]['multipart'] = 'related';
+
+                $this->attachments[$i]['cid'] = uniqid(basename($attachment['name'][1]) . '@', true);
+
+                return $this->attachments[$i]['cid'];
+            }
         }
 
         return false;
diff --git a/tests/system/Email/EmailTest.php b/tests/system/Email/EmailTest.php
index dfcbedf183cc..77f40dead168 100644
--- a/tests/system/Email/EmailTest.php
+++ b/tests/system/Email/EmailTest.php
@@ -162,4 +162,55 @@ private function createMockEmail(): MockEmail
 
         return new MockEmail($config);
     }
+
+    public function testSetAttachmentCIDFile(): void
+    {
+        $email = $this->createMockEmail();
+
+        $email->setFrom('your@example.com', 'Your Name');
+        $email->setTo('foo@example.jp');
+
+        $filename = SUPPORTPATH . 'Images/ci-logo.png';
+        $email->attach($filename);
+        $cid = $email->setAttachmentCID($filename);
+        $email->setMessage('<img src="cid:' . $cid . '" alt="CI Logo">');
+
+        $this->assertTrue($email->send());
+
+        $this->assertStringStartsWith('ci-logo.png@', $cid);
+        $this->assertStringStartsWith(
+            'ci-logo.png@',
+            $email->archive['attachments'][0]['cid']
+        );
+        $this->assertMatchesRegularExpression(
+            '/<img src="cid:ci-logo.png@(.+?)" alt="CI Logo">/u',
+            $email->archive['body']
+        );
+    }
+
+    public function testSetAttachmentCIDBufferString(): void
+    {
+        $email = $this->createMockEmail();
+
+        $email->setFrom('your@example.com', 'Your Name');
+        $email->setTo('foo@example.jp');
+
+        $filename  = SUPPORTPATH . 'Images/ci-logo.png';
+        $imageData = file_get_contents($filename);
+        $email->attach($imageData, 'inline', 'image001.png', 'image/png');
+        $cid = $email->setAttachmentCID('image001.png');
+        $email->setMessage('<img src="cid:' . $cid . '" alt="CI Logo">');
+
+        $this->assertTrue($email->send());
+
+        $this->assertStringStartsWith('image001.png@', $cid);
+        $this->assertStringStartsWith(
+            'image001.png@',
+            $email->archive['attachments'][0]['cid']
+        );
+        $this->assertMatchesRegularExpression(
+            '/<img src="cid:image001.png@(.+?)" alt="CI Logo">/u',
+            $email->archive['body']
+        );
+    }
 }