I'm using Safari 10.0.3 on El Capitan 10.11.6.
The right-click and select "Download Linked File" only works for PDFs for me, not for other file types e.g. .xlsx (Excel files) or .fdx (Final Draft files).
On the Gmail Help Forum topic Bradford started, the workaround that does work for me is the one from Dr. Yavuz Samur: https://productforums.google.com/d/msg/gmail/-jn_07jplD8/YfMagfELBQAJ
In case that link doesn't work, the steps are:
- reply to the original email
- select the "More options" drop-down menu by clicking on the small down arrow in the bottom right of your reply window
- select "Include original attachments"
- right-click on the attachment in your reply window
- select "Download Linked File"
and then discard your reply.