*`file` File - A web [File](https://developer.mozilla.org/en-US/docs/Web/API/File) object.
Returns `string` - The file system path that this `File` object points to. In the case where the object passed in is not a `File` object an exception is thrown. In the case where the File object passed in was constructed in JS and is not backed by a file on disk an empty string is returned.