Since #3365703 was merged, the module is not working anymore if Drupal is installed in a sub folder. For example served under /some/thing instead of directly in /.
This is because in #3365703, the filePublicPath in DownloadManager service is returning the baseUrl and then removing the scheme, host and port, but the baseUrl will also return the basePath. So in the example above, it will remain some/thing/sites/default/files and not sites/default/files as it is expected by the StageFileProxySubscriber when comparing if the requested path is a file or not.
Needs work
3.1
Code