The filechooser gio backend in current libgnomeui displays blank fallback icons for every file, no matters its content (excluding the files with an associated thumbnail, for which the thumbnail appears).
This is a regression from the behaviour of the gnomevfs-based backend; screenshots of the two filechooser will follow.

Looking at the code I pinpointed a possible explanation of this bug: in the function translate_file_info() the gio backend calls gtk_file_info_set_icon_name() and saves in the gchar *icon_name field of the struct GtkFileInfo strings like "application-x-shellscript" or "text-plain"; the gnome-vfs backend on the contrary passes strings like "gnome-mime-application-x-shellscript" and "gnome-mime-application-x-perl" (I'm told that gio supports the new icon naming specification).

Apparently, whatever renders the icons (in gtk+?) doesn't accept the new names. To test it, I hardcoded "gnome-mime-application-x-shellscript" and then "application-x-shellscript"; with the former, the filechooser used a "sh file" icons for every file, in the second it fell back to the generic "blank sheet" icon.