XMS IBytesMessage causing problems with split ZIP file - ibm-mq

Question

Since upgrading MQ to "IBM MQ Explorer V9.1", the XMS libraries that always worked in previous versions have started behaving differently.
Essentially, the code still recognises the messages as IBytesMessage type, and successfully writes them to file via a Byte array, but the file itself, which is a split zip file, fails to reconstitute itself.
Here's the lions share of that code:
Dim FactoryFactory As XMSFactoryFactory = XMSFactoryFactory.GetInstance(XMSC.CT_WMQ)
'Create a ConnectionFactory Object
cfConnectionFactory = FactoryFactory.CreateConnectionFactory()
'this variable will contain the full path of any file downloaded from MQ
Dim strMQMessageOutputFileDestinationFilePath As String = ""
'This variable will be used to evaluate whether the MQ Message Output file exists
Dim fiMQMessageOutputFile As FileInfo = Nothing
'Set various Connection Factory properties
cfConnectionFactory.SetStringProperty(XMSC.WMQ_HOST_NAME, Me.HostName)
cfConnectionFactory.SetIntProperty(XMSC.WMQ_PORT, 1414)
cfConnectionFactory.SetStringProperty(XMSC.WMQ_CHANNEL, "SYSTEM.DEF.SVRCONN")
cfConnectionFactory.SetIntProperty(XMSC.WMQ_CONNECTION_MODE, 1)
cfConnectionFactory.SetStringProperty(XMSC.WMQ_QUEUE_MANAGER, Me.QueueManager)
cfConnectionFactory.SetIntProperty(XMSC.WMQ_BROKER_VERSION, 0)
'Create a new Iconnection object via the Connection Factory
connection = cfConnectionFactory.CreateConnection()
'Create a sesion via the Connection Object, using ClientAcknowledge mode
'ClientAcknowledge is being used because it allows us to control precisely
'when a message should be removed from the queue
session = connection.CreateSession(False, AcknowledgeMode.ClientAcknowledge)
'Create a destination using the Session Object
destination = session.CreateQueue(Me.mstrDestinationURI)
destination.SetIntProperty(XMSC.DELIVERY_MODE, 1)
'Create a consumer using the Session & Destination Objects
Consumer = session.CreateConsumer(destination)
connection.Start()
'IMessage is the base class that is returned from the Consumer's Receive method
Dim recvMsg As IMessage = Nothing
' Retrieve message from Queue
recvMsg = Consumer.ReceiveNoWait
strFileNameFromMsg = If(Not recvMsg.PropertyExists("fileName"), "",
recvMsg.GetStringProperty("fileName"))
If TypeOf (recvMsg) Is IBytesMessage Then
'Binary Message
Dim msg As IBytesMessage = CType(recvMsg, IBytesMessage)
Dim buffer(msg.BodyLength) As Byte
msg.ReadBytes(buffer)
Dim content As String = Text.Encoding.UTF8.GetString(buffer)
'The PrepareDestinationFile Function will generate a unique file name for the new file
'and ensure that the file does not already exist on the drive
strMQMessageOutputFileDestinationFilePath = PrepareDestinationFile(strFileNameFromMsg)
'A FileStream object is needed to write a binary array to a file
Dim fsZipFile As FileStream = New FileStream(strMQMessageOutputFileDestinationFilePath, FileMode.Create)
'Write the contents of the Byte Array to the File via the FileStream object
fsZipFile.Write(buffer, 0, buffer.Length)
fsZipFile.Close()
End If
So, the code doesn't throw any kind of exception - the code still recognises the messages as IBytesMessage, but the files won't unzip correctly.
Oddly, If we use rfhutilc.exe, we can manually pull files provided we set the Write options as No Headers and not Include MQMD - but the code above always worked in the previous version of MQ / XMS
Any assistance you can provide would be very much appreciated.