regedit: Append '.reg' file extension if necessary.

programs/regedit/framewnd.c

@@ -493,6 +493,7 @@ static BOOL ExportRegistryFile(HWND hWnd)
     InitOpenFileName(hWnd, &ofn);
     LoadStringW(hInst, IDS_FILEDIALOG_EXPORT_TITLE, title, ARRAY_SIZE(title));
     ofn.lpstrTitle = title;
+    ofn.lpstrDefExt = L"reg";
     ofn.Flags = OFN_ENABLETEMPLATE | OFN_ENABLEHOOK | OFN_EXPLORER | OFN_HIDEREADONLY | OFN_OVERWRITEPROMPT;
     ofn.lpfnHook = ExportRegistryFile_OFNHookProc;
     ofn.lpTemplateName = MAKEINTRESOURCEW(IDD_EXPORT_TEMPLATE);