Fix bug preventing upload of background images

I made a deploy last night that I didn’t expect to have any visible effect to users (I was changing the location of some static files) so I didn’t make a release post. But unfortunately that deploy made it so users could no longer upload background images. Whoops!

I’ve fixed the issue and you should now be able to upload background images again. Sorry for the inconvenience, hopefully this didn’t catch too many people!