use doctype File directly instead of save_file from file_manager.py

[Mutantpenguin]

save_file will change the filename and not clean up after itself after a rollback

This is a preparation for another (not yet done) PR to solve https://github.com/frappe/frappe/issues/25178 which will completey remove the save_file function from file_manager.py.

[stale[bot]]

This pull request has been automatically marked as inactive because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[Mutantpenguin]

No reason from myself to close it.

[Mutantpenguin]

Please reopen again, it's still valid as far as I know.