Image via WikipediaHelp me brainstorm here, please. What's the problem with using the file manager as the "Open File" dialog? If the file manager respects some basic switches to limit the types of files that are displayed to those that are supported by the calling appliction, couldn't you just call the file manager instead of gtk-open?
Of course, this would be a lot easier in a tag-based file manager, wouldn't it? You'd just call the manager with a search for supported file types and be done with it.