[setr]

Presumably it would be in the same fashion that file dialogs are generated; you defer the actual selection to some OS-provided library function, and just accept a standardized output (some FileObject construct). So the terminal would provide this file selector, based on context: a script would take it as the filename itself, the interactive shell would provide the file dialog, etc.

Terminal Programs are already outputting differently based on context (ie ls to a pipe vs screen), so supporting interactive and scripting is hardly a novel idea in the space; its just that the interactive support hasn't fundamentally improved in decades