I was wondering why are image previews in GtkFileChooser shown only under some circumstances. It turned out that GtkFileChooser doesn't have a builtin image feature; instead, it allows for applications to explicitly add their custom preview widget via
gtk-file-chooser-set-preview-widget() method. Some apps, like GIMP and Firefox, add it, but others don't.
It would be handy to have image previews in any apps using GtkFileChooser. Is there a solution, possibly a preload library like the nocsd hack, that hooks into any application, checks if the application set it's own preview widget, and provides a generic preview widget if it didn't?