Cannot download pdf from Tor Browser using download button

Sine Tails 1.5, when viewing a pdf file in Tor Browser, you can’t download it using the download button of the pdf viewer.
You can achieve it doing right-click -> save as….

It worked in Tails 1.4.1, and the bug has been reproduced on Tor Browser 5.O. In fact it’s already tracked in their own bug tracker:

https://trac.torproject.org/projects/tor/ticket/16781

Original created by @mercedes508 on 10053 (Redmine)