fix(forumchannel): set attachment data correctly

[shiftinv]

Summary

When creating a forum thread, the "attachments" field was previously being added to the payload at the top level, instead of in the nested "message" field.
This prevented creation of forum threads with just files (works fine if you add embeds, content, or anything else), and also meant that file descriptions wouldn't be sent if you hadn't already hit the first issue.

To reproduce:

await forum_channel.create_thread(
    name="attachment test",
    file=disnake.File("/path/to/file"),
)

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)