Jan 24, 2016
UPLOAD STATIC FILE
Upload Static File page is gone, and tumblr’s redirecting you to your customize page. That’s because you can use another way to upload static files for your themes.
1) Edit HTML
Goto your customization page, and click Edit HTML.
2) Theme Assets
Click the Settings Screw on top, and choose Theme assets.
3) Add A File
Choose your theme stuff for upload, like js/css/png/jpg/etc…
4) Insert the Link. Now do what you want with it
Find the uploaded file, and click Insert, and you should now have a link to the file in your editor. Cut the link and do what you wanted to with it.
(SOURCE: http://eugenekoh12.tumblr.com/post/128635591425/as-you-all-may-know-im-late-i-know-but-the)