- Enhanced the `mailer` provider to support email attachments, including both regular and inline attachments.
- Updated the `buildMessage` method to handle multipart email formats for messages with attachments.
- Added new tests for attachment handling in `mailer_test.go` and `mailgun_test.go`, ensuring comprehensive coverage for single, multiple, and inline attachments.
- Revised `TODO.md` to reflect the completion of attachment support across email providers, confirming that all major providers now support this feature.