Unexpected SerializationException when using IRequiresRequestStream - deserialization

Question

We have a request Dto which is defined as this:
public class AddDocumentByUploadBinaryRequest: AddDocumentRequest, IRequiresRequestStream
{
public string FileName { get; set; }
public Stream RequestStream { get; set; }
}
And AddDocumentRequest class definition is:
public class AddDocumentRequest: IReturn<ResponseBase>
{
public Guid Id { get; set; }
public string[] Tags { get; set; }
public Guid? LotId { get; set; }
public Guid? ContactId { get; set; }
// never need it to attach document to existing bill
//public Guid? BillId { get; set; }
public Guid? FolioId { get; set; }
public Guid? JobTaskId { get; set; }
public Guid? TaskId { get; set; }
public Guid? TenantInvoiceId { get; set; }
public Guid? InspectionTaskId { get; set; }
public Guid? InspectionReportAreaId { get; set; }
public Guid? InMessageId { get; set; }
public DocumentTypes? DocumentType { get; set; }
public EventLinkTypes? DocumentArea { get; set; }
public Guid? MessageId { get; set; }
public Guid? LinkId {
get {
switch (DocumentArea) {
//case EventLinkTypes.Bill:
// return BillId;
case EventLinkTypes.Contact:
return ContactId;
case EventLinkTypes.Inspection:
return InspectionTaskId;
case EventLinkTypes.InspectionReport:
return InspectionTaskId;
case EventLinkTypes.InspectionReportArea:
return InspectionReportAreaId;
case EventLinkTypes.Job:
return JobTaskId;
case EventLinkTypes.Lot:
return LotId;
case EventLinkTypes.Task:
return TaskId;
case EventLinkTypes.Message:
return MessageId;
default:
return FolioId;
}
}
}
}
The API works most of the time but occassionally, we get "Unable to bind to request" error with the details like this:
Unable to bind to request 'AddDocumentByUploadBinaryRequest'; Error code: SerializationException; Field name: ���iD�����; Message: '�Z~�l�_$�6j�Ǒ�Xƾ��ԯ㵂���;W;/�����k� 6��;ׯG$�O�`�V�j��a�Z�[S幉>�>����[An��O
X�n$�<�p�H��GN��[���c�|5�#��
?�' is an Invalid value for '���iD�����'
The captured error message suggests that the binary body was unexpectedly deserialized and failed.
The client device always uses Content-Type: application/octet-stream header. I have read relevant source code of ServiceStack but I couldn't think of why the binary body would still go into deserialization.
Any ideas would be appreaciated.