Where exactly do you double click on files? When you receivea file in Spark it should show you Open and Open Folder links-buttons. Do you mean it can’t open file when you press Open? Can you still press Open Folder and then open the file?
I don’t think it works this way. I think Spark is only redirecting query to the operating system/desktop environment and then OS itself decides how to open it. So probably Spark is not informing KDE/Linux the right way.