feat: add inline attachment support to EmailEndpoint

Introduced optional `contentId` for inline email attachments, enabling embedding images or files directly in HTML emails. Updated tests to validate the new functionality, ensuring correct handling of `contentId`. Enhanced the README with examples demonstrating usage of inline attachments.

Author: bjarn

README.md

@@ -56,11 +56,26 @@ $lettermint->email
     ->text('Hello!')
     ->headers(['X-Custom-Header' => 'Value'])
     ->attach('document.pdf', base64_encode($fileContent))
+    ->attach('logo.png', base64_encode($logoContent), 'logo@example.com')
     ->route('my-route-id')
     ->idempotencyKey('unique-request-id-123')
     ->send();
 ```
 
+#### Inline Attachments
+
+You can embed images and other content in your HTML emails using content IDs:
+
+```php
+$lettermint->email
+    ->from('sender@example.com')
+    ->to('recipient@example.com')
+    ->subject('Email with inline image')
+    ->html('<p>Here is an image: <img src="cid:logo@example.com"></p>')
+    ->attach('logo.png', base64_encode($imageContent), 'logo@example.com')
+    ->send();
+```
+
 ### Idempotency
 
 To ensure that duplicate requests are not processed, you can use an idempotency key:

src/Endpoints/EmailEndpoint.php

@@ -134,14 +134,21 @@ public function replyTo(string ...$emails): self
      *
      * @param  string  $filename  The attachment filename.
      * @param  string  $base64Content  The base64-encoded file content.
+     * @param  string|null  $contentId  Optional content ID for inline attachments.
      */
-    public function attach(string $filename, string $base64Content): self
+    public function attach(string $filename, string $base64Content, ?string $contentId = null): self
     {
-        $this->payload['attachments'][] = [
+        $attachment = [
             'filename' => $filename,
             'content' => $base64Content,
         ];
 
+        if ($contentId !== null) {
+            $attachment['content_id'] = $contentId;
+        }
+
+        $this->payload['attachments'][] = $attachment;
+
         return $this;
     }
 

tests/Endpoints/EmailEndpointTest.php

@@ -157,6 +157,34 @@
         ->send();
 });
 
+test('it handles attachments with content_id', function () {
+    $attachment = [
+        'filename' => 'logo.png',
+        'content' => 'base64encodedimage',
+        'content_id' => 'logo@example.com',
+    ];
+
+    $this->httpClient
+        ->shouldReceive('post')
+        ->once()
+        ->with('/v1/send', [
+            'from' => 'sender@example.com',
+            'to' => ['recipient@example.com'],
+            'subject' => 'Test Subject',
+            'html' => '<img src="cid:logo@example.com">',
+            'attachments' => [$attachment],
+        ], [])
+        ->andReturn(['message_id' => '123', 'status' => 'pending']);
+
+    $this->endpoint
+        ->from('sender@example.com')
+        ->to('recipient@example.com')
+        ->subject('Test Subject')
+        ->html('<img src="cid:logo@example.com">')
+        ->attach('logo.png', 'base64encodedimage', 'logo@example.com')
+        ->send();
+});
+
 test('it handles custom headers', function () {
     $this->httpClient
         ->shouldReceive('post')