Currently the list of extensions which is used for "browsing" with the "next" and "previous" buttons/keys is automatically generated based on known extensions for mime types within GTK+. While this is sufficient in most cases, this list is not always complete. For example, we have files with an extension of ".psi", which are actually JPEG files. Based on their mime type, they are correctly recognized and opened by GPicView. However they are not browsable: the "next" and "previous" actions don't do anything in this case (provided no other, recognized, images are in the same directory of course).
My suggestion is that a text box be added in the preferences window for custom file extensions, and to save this list in the config file in ~/.config/gpicview/gpicview.conf. By default, the list should probably be empty.