The thing is that although this may be a little more self-explanatory, it is more complicated. It breaks the decision about what to do with an attachment into multiple logical steps: What do I want to do to the attachment? If I'm opening it, what program do I want to do it with? And am I just reading it or am I expecting to change it? Still, that's a lot of decisions to present to a user.
IBM has lots of smart people doing this kind of thing as a full-time job. They have experienced UI designers and they have usability test facilities. Anything that we bloggers and blog-readers can think of on the spur of the moment has almost certainly been given very serious consideration by IBM. Deciding what to do with a file attachment is not a simple matter, and there's never going to be one dialog design that is best for all users in all situations in all applications. I'm done with worrying about this for now. I'll stick with my suggestion that IBM put the contents of the dialog under the control of application developers in the long-term, but for the short term I'm going to trust that what they're doing in 7.0 is neither better nor worse than any other alternatives.