not sure if OpenCV can read .mpg files, but no harm in listing it

panda/src/grutil/config_grutil.cxx

@@ -72,7 +72,7 @@ init_libgrutil() {
   PandaSystem *ps = PandaSystem::get_global_ptr();
   ps->add_system("OpenCV");
   TexturePool *ts = TexturePool::get_global_ptr();
-  ts->register_texture_type(OpenCVTexture::make_texture, "avi");
+  ts->register_texture_type(OpenCVTexture::make_texture, "avi mpg");
 #endif // HAVE_OPENCV
 }