If you have already successfully upgraded your database from 10 to 14 and the only thing still outstanding is your attachments, then you should simply move your filestore as you would move any set of folders.
It seems... unlikely... that you would have successfully upgraded from 10 to 14 without knowing this however so maybe there's more to the question than I'm seeing?
For general knowledge, check out the Odoo 14 Documentation on importing documents:
If you have already successfully upgraded your database from 10 to 14 and the only thing still outstanding is your attachments, then you should simply move your filestore as you would move any set of folders.
It seems... unlikely... that you would have successfully upgraded from 10 to 14 without knowing this however so maybe there's more to the question than I'm seeing?
For general knowledge, check out the Odoo 14 Documentation on importing documents: