You have to migrate the attachment files manually. Otherwise, consider purchasing a 3rd party conversion service. You can request to migrate the attachment files as the additional work. Contact us in private for details.

I would recommend you are keep using QB Desktop. If you are running an outdated QB Desktop version on Win 8.1 or 10, consider upgrading to QB Desktop 2019 with a one time license to save your cost. Deploy a private cloud to access your file by internet.