Add contentId method

Author: stevebauman

readme.md

@@ -458,12 +458,12 @@ To conveniently work with attachments, the `Message` class includes methods that
 
 For each attachment, you may access the following properties:
 
-- `$attachment->filename(): string`: Retrieve the attachment's filename.
+- `$attachment->filename(): string|null`: Retrieve the attachment's filename.
 - `$attachment->contents(): string`: Retrieve the attachment's contents.
+- `$attachment->contentId(): string|null`: Retrieve the attachment's content ID (cid).
 - `$attachment->contentType(): string`: Retrieve the attachment's content type.
 - `$attachment->contentStream(): StreamInterface`: Retrieve the attachment's contents as a stream.
 
-
 > [!important]
 > The attachment's content type is determined by the `Content-Type` header provided in the email, and may not always be accurate.
 > If `application/octet-stream` is returned, using [`mime_content_type()`](https://www.php.net/manual/en/function.mime-content-type.php) over the filename or contents may be used to clarify the type.