GtkFileChooser::set_preview_widget
void set_preview_widget(GtkWidgetpreview_widget);
Sets an application-supplied widget to use to display a custom preview of the currently selected file. To implement a preview, after setting the preview widget, you connect to the update-preview signal, and call get_preview_filename() or get_preview_uri() on each change. If you can display a preview of the new file, update your widget and set the preview active using set_preview_widget_active() . Otherwise, set the preview inactive.
When there is no application-supplied preview widget, or the application-supplied preview widget is not active, the file chooser may display an internally generated preview of the current file or it may display no preview at all.