You just said, "No one forces you to use their proprietary software." and I gave an example. Of course, it is not forcing, you can stay not opening the attachment, suppress the curiosity.

Yes, file formats /can/ be seen as a different issue, but its often the reason I end up using proprietary programs.

One can have a closed program with a open file format, but not an open program with a proprietary file format, why? Doesn't it suggest an interesting link between proprietary programs and proprietary file formats? Do you still think they are really independent issues?