Please do not try scanning the widget tree for the GtkFileChooser to work out a place to put your own widgets. This is totally undocumented, so if you do get it to work it will probably break unexpectedly in the next version of GTK, even a minor version change in GTK could do this.
The only ways to add extra widgets to the GtkFileChooser widget is to use " gtk_file_chooser_set_preview_widget() " if the style of widget is like that of a preview to the selected file. It even has a signal to indicate if there is a need to change the preview.http://developer.gnome.org/gtk3/stable/GtkFileChooser.html#gtk-file-chooser-set-preview-widget
The other method is to use " gtk_file_chooser_set_extra_widget () " this will then add an extra widget, say for other file options.http://developer.gnome.org/gtk3/stable/GtkFileChooser.html#gtk-file-chooser-set-extra-widget
Remember with these methods that the passed widget could be a container which then can have many other widgets inside them.