You might find that even though you've installed Java, uploading multiple files still isn't working for you. This is usually because Java hasn't installed properly due to User Account Control on Windows machines, and the easiest way to get this working is to perform the following steps:

1) Close all currently open browser windows
2) Locate the icon to load your browser of choice (usually on your desktop)
3) Right click on the icon and select to "Run as Administrator"
4) Once your browser is open, navigate to http://www.java.com/en/download/installed.jsp
5) A message will more than likely appear saying that you're not using the correct version of Java. Select to update Java to the latest version
6) Once that's done, go back into Huddle and attempt to upload your files again