I tested the 30 second skip here and it appeared to work fine, possibly the file you are trying it on is being transcoded on the fly by the Mezzmo server on your PC?

Instead of downloading the file you can push the URL directly to the Chromecast so that it plays it without downloading to your Android device, when you select to download the file in your browser if you are prompted to select an app to handle the download you can select Mezzmo and this will allow playing the file directly to the chromecast.