function getCurrentlyViewedFile(paneId) {
        var pane = _getPane(paneId);
        return pane ? pane.getCurrentlyViewedFile() : null;
    }
Retrieves the currently viewed file of the specified paneId
@param {?string} paneId - the id of the pane in which to retrieve the currently viewed file
@return {?File} File object of the currently viewed file, or null if there isn't one or there's no such pane