One can, but it's incredibly unobvious what to do. That 6 KB is the HTML
saying "what you're trying to download is in the sandbox, so you have to
log in to addons.mozilla.org before you can get the actual file." Thanks
to bug 375622, the dictionary page will list the newest version of any
dictionary, even if that version is in the sandbox and there is an older
version which is not. So the only thing you can do, as a user who just
wants a dictionary which is in that state, is to create a developer
account on AMO (which, fortunately, doesn't require much from you), so
that you can log in before saving the file, so that you'll actually save
the file instead of some HTML.