Web Messaging Guest API file uploads returning 415 on standard MIME types

Piping customer attachments into our Rust ingestion service via the Guest API. The docs point to POST /api/v2/conversations/messaging/contacts/{contactId}/messages with multipart/form-data, but the gateway keeps throwing a 415. We’re sending image/png and application/pdf under 5MB. Does the Web Messaging channel enforce a strict MIME whitelist server-side? Here’s the builder:

client.post(url).multipart(Multipart::new()
 .text("messageType", "file")
 .file("attachment", path).unwrap())
.send().await?;

The tokio task just hangs waiting for a response frame.