Gmail always gives me trouble when trying to download an attachment. Just now, I couldn’t download my attachment for no reason. There was no error message, it just wouldn’t make a download button.
If you want to skip it, pretend you’re forwarding the message and the attachment will appear as an attachment that you can just click to reliably download it.
Yuji Tomita 
Unfortunately, this doesn’t work anymore… What you can do, is forward the message to another address.
Hmm, I just tested it and it works for me. Click forward and you can download the attachment!