Ian, in this case, Kerry is talking about the setting in the Upload File behavior for If the File Exists.
Not the binding in the data Assist insert record behavior.
in the insert record behavior, the server file name binding should be used, but do not change the If File Exists setting in the upload file behavior to use [Server File Name]. The If file exists setting is used to determine the server file name, so server file name cannot be used as a part of that setting, it would create circular logic.