windeployqt missing file when using multimedia

Bug could reproduce like this:

1. Build Qt example Media Player Example with release config, get player.exe;
2. Run windeployqt player.exe --release, get all dlls done;
3. Double click player.exe, nothing happened. The application not run, neither show errors;
4. copy plugins/multimedia/ folder to dir that player.exe in, double click again, it runs successfully.

Apparently it's a kind of missing I think.