Posts a message on a dispute, optionally with files as evidence.

To attach files:

1. Declare each file in attachments with its mimeType, fileName and size.
2. The response returns one upload per file. Send the file with a PUT to its uploadUrl, with the uploadHeaders and a Content-Type header set to the file's mimeType.
3. A file appears on the thread once its upload completes.

Errors

• 400 VALIDATION_ERROR: body is missing, blank or over 2000 characters, or an attachment is invalid
  (DISPUTE_EVIDENCE_INVALID, DISPUTE_EVIDENCE_TOO_MANY, DISPUTE_EVIDENCE_MIME_INVALID, DISPUTE_EVIDENCE_TOO_LARGE)