attachment.php header problem

[JamesAB]

You can see this problem right here on vbulletin.org

Just click on on any image attachment that was uploaded here and then try to save the attachment. Rather than seeing the image's actual file name, you'll get "attachment.php" as the file name in Internet Explorer (unless you have already set your browser to view vbulletin.org in "compatibility" mode) or while using Firefox 39. Earlier versions of Firefox (38 and below) work as you would expect and you see the attachment's real file name in the "Save As" dialog box.

This is an annoying problem for a forum with lots of image attachments that people want to save to their local computers, so I hope someone can share some insight on how to fix this.

Thanks for your help,
James