Installed the DOpus 12 on Window 10 64-Bit. I found that I cannot directly drag the attachment of the email (by Foxmail) to the DOpus for saving. But I can make it to the desktop or the default explorer.
If you're using Opus Pro, please try setting Preferences / Miscellaneous / Advanced: dragdrop_async = False to see if it works then.
Please let us know if that works as we may be able to make it automatic for the program in question.