Using kmail 4.8.3, when clicking on an email attachment, the "open with" dialog appears. I have to right click and select "open" for it to open with the default program. Is there a way to change this to make open the default option when clicking?

I'm not sure, but if you can, please try 4.9 if you are able to, or 4.8.5 when they are out. I think a change to this behavior was committed (but I can't find it now).
In case it's still like this, please open a bug on bugs.kde.org.