This couldn't be done in the official AC package, because it relies on a numbering system to work out what slot the save goes into (e.g. "MySave_3.save").
But to add this in your own package, look into modifying SaveSystem.cs' GetSaveFileName function. It's a simple function to work out the expected filename based on the slot number.
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But to add this in your own package, look into modifying SaveSystem.cs' GetSaveFileName function. It's a simple function to work out the expected filename based on the slot number.