It's my understanding that you have an attachment that's not opening in Mail. I may be able to help with this. How big is the attachment? The message you've is showing that the file is downloading which will need to be done first before it's opened. This can depend on your connections speed and the email service provider as well. I'd let it continue to download and see if it finishes. You could also help narrow down the issue by going to the email service provider web site and log in using the web browser. From there try seeing if you can download the attachment that way.