we are working on a data migration project between two salesforce platform. The legacy salesforce is still in classic and it has 3000 + attachment to either case or account object. The goal is to migrate the object with its attachment and store it as file in the lightning version. I tried SFDMU tool and enable (both org) upload file as File not Attachment settings. However, it still shows as attachment in target org which is not searchable and showing on the UI.
we try to avoid to install any new management package or application in the legacy org, the reason we pick SFDMU because the developer can install and run locally.

Can anyone provide some guidance? Thank you so much!