post
https://mapi-sandbox.unimaze.com/api/v1/messages//documents
Attaches a document to an existing message.
You can also submit the data as a stream that is formatted as a base64-encoded ASCII text.
In this case, the HTTP Content-Type header must be set to application/base64.
The document must be sent as the request body.
When the data is uploaded, the call then informs the server of whether more data is expected or not and updates the message status.
If no more documents will be added, the call sets the status of the message to received.
Otherwise, the message remains open and the message status is notset.
The call throws an exception if the message does not exist or if it has already been closed, which means that its message status is received.