Hi,
It's not feasible to make "attach as a copy" the default attach option when inserting cloud attachments. However, it sounds a good idea to Outlook 2016. I'd recommend you submit a feedback to Microsoft via File > Feedback > Outlook 2016 Feedback.
Microsoft values all customer feedback and the production team will consider whether to add the feature according to customer needs.
Thank you for your understanding and support.
Regards,
Steve Fan
TechNet Community Support
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