procedure TMainDataModule.NXDBSession1RemoteSendProgress(Sender: TObject;
  NumBytes: Integer; PercentDone: Word);
begin
  if MvSendingFile then begin
//    MLProgress1.Position1 := PercentDone; << This eventually fails 
after some uploads
    MLProgress1.Caption1 := IntToStr(NumBytes)+'kb'; << this will work OK
  end;
end;
*********************

And this is the example code I use to upload the file to the Blob:
*********************
procedure TEmbarquesForm.Insert2Click(Sender: TObject);
begin
    if OpenDialog1.Execute then begin
      MainDataModule.MvSendingFile := True;
      MainDataModule.MLProgress1.Show;
      MainDataModule.MLProgress1.Title := 'Archivando Documento';
      DBTableEmbarquesDocumentos.Append;
      DBTableEmbarquesDocumentosArchivo.LoadFromFile(OpenDialog1.FileName);
      DBTableEmbarquesDocumentos.FieldByName('TipoDocumento').AsString 
:=  ExtractFileName(OpenDialog1.FileName);
      DBTableEmbarquesDocumentos.FieldByName('ArchivoOriginal').AsString 
:=  ExtractFileName(OpenDialog1.FileName);
      try
        DBTableEmbarquesDocumentos.Post;
      finally
        MainDataModule.MvSendingFile := False;
        MainDataModule.MLProgress1.Hide;
      end;
    end;
end;