GTK: Fix a warning for file->open dialogs.

SetCurrentName only applies to save dialogs, so call it conditionally.

src/platform/guigtk.cpp

@@ -1295,14 +1295,17 @@ public:
         }
     }
 
+//TODO: This is not getting called when the extension selection is changed.
     void FilterChanged() {
         std::string extension = GetExtension();
         if(extension.empty())
             return;
 
         Platform::Path path = GetFilename();
+        if(gtkChooser->get_action() != GTK_FILE_CHOOSER_ACTION_OPEN) {
             SetCurrentName(path.WithExtension(extension).FileName());
         }
+    }
 
     void FreezeChoices(SettingsRef settings, const std::string &key) override {
         settings->FreezeString("Dialog_" + key + "_Folder",