I think it's not a problem to squash that bug, but please do it, it's rather annoying.
1. I want to open an attachment from TheBrain:
TheBrain asks me what application I want to use. That's OK.
2. I fill in the line with the appropriate command:
I hit Enter and the attachment opens as it should be.
3. If I want to open attachment of that kind again, TheBrain presents me with the form in which the name of application I used to open the attachment with is already present:
4. But if I close TheBrain and start it again, it completely forgets about everything: