I have the same problem description here. I solved this problem by adding the following Javascript code to the aspx page:
window.opener = null;
window.open ('','_self');
window.close();
The new page opens and pops up a filedownload dialog and the page closes. But I don't understand why the Javascript code executes. The response type is application/octet-stream not text/html and the fiddler prove this. Can anyone explain this to me?