feat: add support for inline attachments in email requests

- Extended `attach` method to support optional `content_id` for inline attachments.
- Updated tests to verify content_id functionality.
- Updated README with documentation and examples for inline attachments.

Author: bjarn

README.md

@@ -55,6 +55,7 @@ const response = await lettermint.email
     'X-Custom-Header': 'Custom Value',
   })
   .attach('attachment.txt', Buffer.from('Hello World').toString('base64'))
+  .attach('logo.png', Buffer.from('...').toString('base64'), 'logo') // Inline attachment
   .idempotencyKey('unique-id-123')
   .metadata({
     foo: 'bar',
@@ -90,7 +91,7 @@ Methods for sending emails:
 - `bcc(...emails: string[])`: Set one or more BCC email addresses
 - `replyTo(...emails: string[])`: Set one or more Reply-To email addresses
 - `headers(headers: Record<string, string>)`: Set custom headers for the email
-- `attach(filename: string, base64Content: string)`: Attach a file to the email
+- `attach(filename: string, base64Content: string, content_id?: string)`: Attach a file to the email. Optional `content_id` for inline attachments.
 - `route(route: string)`: Set the routing key for the email
 - `idempotencyKey(key: string)`: Set an idempotency key to prevent duplicate email sends
 - `metadata(metadata: Record<string, string>)`: Set metadata for the email

src/endpoints/email.spec.ts

@@ -187,6 +187,28 @@ describe('EmailEndpoint', () => {
     });
   });
 
+  it('should add attachments with content_id', () => {
+    const result = emailEndpoint.attach('image.png', 'base64imagedata', 'logo');
+
+    expect(result).toBe(emailEndpoint);
+
+    return emailEndpoint.send().then(() => {
+      expect(client.post).toHaveBeenCalledWith(
+        '/send',
+        expect.objectContaining({
+          attachments: [
+            {
+              filename: 'image.png',
+              content: 'base64imagedata',
+              content_id: 'logo',
+            },
+          ],
+        }),
+        undefined
+      );
+    });
+  });
+
   it('should set the route', () => {
     const result = emailEndpoint.route('test-route');
 

src/endpoints/email.ts

@@ -13,6 +13,11 @@ export interface EmailAttachment {
    * The base64-encoded content of the attachment
    */
   content: string;
+
+  /**
+   * The Content-ID for inline attachments (optional)
+   */
+  content_id?: string;
 }
 
 /**
@@ -279,16 +284,18 @@ export class EmailEndpoint extends Endpoint {
    *
    * @param filename The attachment filename
    * @param content The base64-encoded file content
+   * @param content_id The Content-ID for inline attachments (optional)
    * @returns The current instance for chaining
    */
-  public attach(filename: string, content: string): this {
+  public attach(filename: string, content: string, content_id?: string): this {
     if (!this.payload.attachments) {
       this.payload.attachments = [];
     }
 
     this.payload.attachments.push({
       filename,
       content,
+      ...(content_id && { content_id }),
     });
 
     return this;