Open galleryserverpro.config and make sure the silverlightFileTypes is configured like this:
Code:silverlightFileTypes=".mp3,.wma,.wmv,.asf,.asx,.mp4,.m4a"
There is an issue where the upgrade wizard imports the settings from your 2.2 version of gsp.config, and that version does not have .mp4 listed in the silverlightFileTypes settings.
Roger Martin
Creator and Lead Developer of Gallery Server Pro