BaseAttachment filename can be undefined

When an email bounces, the receiving mail server often sends back a “delivery status notification” email. That email is usually multipart MIME, with pieces like:

• message/delivery-status: structured delivery failure details, such as failed recipient, SMTP status code, reason.
• message/rfc822: a copy of the original email or original headers.
• message/global-headers: header-only metadata.

These parts are not normal user attachments like invoice.pdf or image.png. They are technical sub-parts of the bounce email, and mail providers often expose them through the same attachments API even though they do not have a filename. Nylas returns filename: undefined.

Start by locating the BaseAttachment declaration and the attachment response types. Trace how MIME parts such as message/delivery-status and message/rfc822 are represented, then verify that an absent filename is handled consistently. Done means these technical parts can be represented without requiring a filename, with the relevant type or behavior covered by the existing attachment checks.