Updates the Create New Message API documentation to explain how to send
file attachments with multipart form data.
Closes#10472Closes#12672
## Why
The endpoint already accepts attachment uploads, but the published API
docs only described the JSON request body. That made it unclear that
clients need to use multipart form data and send files through the
`attachments[]` field.
## What changed
- Adds `multipart/form-data` as a documented request body option for
Create New Message.
- Documents the `attachments` binary array and form encoding used by
`attachments[]`.
- Adds a multipart cURL request example for a message with an
attachment.
- Regenerates the Swagger JSON artifacts.
## Screenshots
Create New Message API docs with the multipart cURL sample and
`multipart/form-data` request sample selected:
<img width="1702" height="1083"
alt="create-message-attachment-docs-multipart"
src="https://github.com/user-attachments/assets/5fef9082-6eb9-494d-91d1-8dc0b7880212"
/>
## How to test
1. Open `/swagger`.
2. Navigate to Application -> Messages -> Create New Message.
3. Confirm the endpoint documents both `application/json` and
`multipart/form-data`, including the attachment payload schema.
