[SHIMGVW] Add file extension on file save (#5975)

In order to add a filename extension if necessary,
set "png" to OPENFILENAME.lpstrDefExt.
CORE-19222

dll/win32/shimgvw/shimgvw.c

@@ -435,8 +435,9 @@ static void pSaveImageAs(HWND hwnd)
     sfn.hInstance   = hInstance;
     sfn.lpstrFile   = szSaveFileName;
     sfn.lpstrFilter = szFilterMask;
-    sfn.nMaxFile    = MAX_PATH;
+    sfn.nMaxFile    = _countof(szSaveFileName);
     sfn.Flags       = OFN_EXPLORER | OFN_OVERWRITEPROMPT | OFN_HIDEREADONLY;
+    sfn.lpstrDefExt = L"png";
 
     c = szFilterMask;