Themes stop working if a child theme contains assets

[dax (Daniela)]

Reported here

Can't upload theme from file

I just tried to import a theme from a file. When I select “from my device” I get the selector and choose the file, but then the “upload” button doesn’t do anything. I tested on two instances Discourse 1.9.0.beta13 and Discourse 1.9.0.beta14. Tested Chome and Firefox on Linux.