I'm not sure which file you saved? But if Microsoft ever pull the free upgrade, there's some risk that "saving the file" approach won't work.
The way I believe W10 activation works is that it once a computer has valid activation for W10, it saves a hardware signature of the computer on its servers. When you install W10 in future, it checks the computer's hardware signature against the list on its servers - and if it's on the list as having previously activated with the same W10 edition, it reactivates.
You can of course buy a W10 licence, and also if the computer was bought with W10, it will have a key on the motherboard somewhere.
But for computers upgraded from W7/8/8.1, if you wanted to "save" an activation, I think the way to do it is
- Fully Backup your setup with W7 (or 8.1, no-one should be running W8.0 now)
- Upgrade to W10
- Make sure it activates,
- Ideally login with an Admin-level Microsoft Account which saves some details in your account and gives more options if hardware changes
- Restore to W7 (or whatever)
Then if you did want to migrate to W10 in future, the hardware signature should be on the servers and activate.