From The Apple Wiki

This is an empty hidden file in the root of iOS. According to Apple, .file can be used to make a “reference” to a file like: /.file/id=6571367.2773272, where the id is a composite key of device ID and inode ID. This acts as a stable identifier to reference a file by its inode, rather than the filename and path that can change if renamed/moved by the user. It is believed to be a fundamental part of how aliasing works.

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