With Jupload and Jdownload, I can see if you don't want to sign it if it's completely open source. I'll give you the benefit of the doubt on that one. My only recommendation is that you make those opt-in components of your server software rather than including them with your installation package. That would give you the opportunity to put a disclaimer saying they're self signed.
WingDownloader though, is a Java application that you put your name on. If you're putting your name on it, then I assume that you consider it your own. It isn't available for download like the other Web Client Plugins, which makes it seem like it's more of an integral part of the Web Client, rather than an optional Web Client Plugin.
You advertise that your web client allows you to transfer multiple files at the same time. The only way to do this (that I'm aware of) is with one of these 3 plugins. Yes, you can lower your security to permit running the application, but I wouldn't expect that from software that I have purchased. Soon, we aren't even going to have the option to lower our Java security to medium. With Java 8 update 20, which will soon be the default download on java.com, the medium security option is removed.
With the security adjustments to Java, you are forcing us to not only purchase your software and the subsequent update protection (if we want to keep the software up to date), but to also purchase a Java Code Signing Certificate. I'm sure your customers aren't looking to distribute this software, as you are the ones distributing it, so this just seems like an unnecessary expense to everyone who has purchased your software expecting it to work properly.
Do us all a favor and sign your code.