I have noticed the same problem, every time that is attempted to download an attached file although posted with the correct name it comes downloaded with name "attachment". If test to change name and to put original name , it works. How it is possible to download directly with the true name (e.g. name.txt or name.zip)?
/edit :
However i've just tried with firefox on gentoo linux and it's ok... It's a Internet Explorer's problem....

Any Solution? Content-mime types incorrect?