Of course, your best course of action if you aren't expecting a message to have an attachment is always to not open that attachment and instead call the sender directly to verify it. While Google has very, very good spam and phishing detection, it's not perfect, and the last line of defense is you.
|The message you'll see once your files are encrypted.|
According to a number of posters on a month-old reddit thread detailing the malware, paying the $300 ransom does work and you will be provided with a key that will decrypt all your files.
The advantage of Google Apps
But it would be much easier to simply not have to worry about this kind of thing at all, wouldn't it? Thankfully, Google Apps protects you from this kind of attack by blocking the sending or receiving of any and all .exe files, even those contained in archived zip files. The best defense is, as always, staying vigilant, but it's nice to know that you don't have to worry about opening any suspicious .exe files, since Google stops them from ever getting to you.
Can your current email system do that? If it can't, it might be time to consider switching to one that allows you to focus on the important things and not worry about whether or not every message you receive contains a hidden malware payload.