Sidecar files don't work at Danbooru website. Mass uploader opened in new tab = fail

[ProximaNova]

Using Firefox for all of the following

As of this revision: #30 (comment) plus line 372 to line 445 changes AND at least one older revision:
. Sidecar files don't work at a Danbooru website, see:
.. #29
.. #30

As of this revision: #30 (comment) plus line 372 to line 445 changes:
. If you are using a Danbooru website and click the mass upload link so that it opens in a new tab, one of two things will happen:
.. The page will only say "not found"
.. The mass uploader page will load but when you try to upload anything you will get this--"Couldn't upload examplefilename.jpeg: error, access denied. Try logging in. Stopped."--the only way to get it working again is open the websites homepage in a new tab then click mass upload so it opens in the tab you are on.