Guest API file upload MIME restrictions and size limits?

Hey folks.

We are implementing file uploads in our custom C# Web Messaging backend using the Guest API. The docs show the endpoint for sending messages, but I can’t find clear specs on accepted MIME types or the hard size limit for attachments.

When I POST a 5MB PDF to /api/v2/conversations/messaging/conversations/{id}/events/message, I get a 400 Bad Request with "error": "Invalid attachment size".

Is there a specific header or payload structure I’m missing? Or is the limit just under 5MB?