Theming: logo and background images don't work with apps outside server root

[kyrofa]

Steps to reproduce

1. Move /var/www/nextcloud/apps to /var/www/apps.
2. Update config.php to include:

'apps_paths' =>  
  array (
    0 =>  
    array (
      'path' => '/var/www/apps',
      'url' => '/apps',
      'writable' => true,
    ),  
  ), 

3. Set alias in httpd.conf:

Alias /apps /var/www/apps

Expected behaviour

Theming should work as normal.

Actual behaviour

Spinners just spin forever when setting the logo/background image, preview doesn't display, and settings don't actually seem to take hold (log out and images aren't set).

Server configuration

Operating system:

Ubuntu 16.04

Web server:

Apache 2.4

Database:

sqlite

PHP version:

7.1

Nextcloud version: (see Nextcloud admin page)

12.0.5 (or master). This issue was introduced in v12, and still exists in v13. Would very much like to backport to 12.0.6 and 13.0.1.

Updated from an older Nextcloud/ownCloud or fresh install:

Fresh

Where did you install Nextcloud from:

Source