Well, I figured it out: tap and HOLD on the emailed attachment, until you get an "Open in FileMaker Go" button. Tap that button and it opens. I think it should work when you tap on the icon of the attachment, like a PDF or a Word document do.
I figured it out by watching the FileMaker Go demo infomercial again. At 2:23 it shows an attachment being opened. However, since in the infomercial the "Open in Filemaker Go" button just sort of magically appears at the bottom of the email-- there's no finger shown-- it took some head scratching to figure it out. Yes, I believe that is somewhat standard behavior for an attachment, but why not show a finger doing the tapping? (And why not just open the file when I tap it, rather than require this tap-and-hold-and-then-tap-the-button method?)