public Task Invoke(IDictionary<string, object> env)
        {
            var request = new OwinRequest(env);
            var response = new OwinResponse(env);

            var requestPath = Uri.UnescapeDataString(request.Path);

            var backgrounds = DataManager.ActualBackgroundStorage;

            if (requestPath.StartsWith("/backgrounds/"))
            {
                if (request.Method != "GET")
                    return response.RespondMethodNotAllowed();

                var query = requestPath.Substring("/backgrounds".Length);

                if (query.EndsWith("/list"))
                {
                    string path = query.Substring(0, query.Length - "list".Length);
                    var dir = backgrounds.GetDirectory(path);

                    try
                    {
                        StringBuilder sb = new StringBuilder();
                        ListBackgroundEntries(dir, sb);

                        return response.RespondString(sb.ToString());
                    }
                    catch (FileNotFoundException)
                    {
                        return response.RespondNotFound();
                    }
                }
                else if (query == "/listall")
                {
                    StringBuilder sb = new StringBuilder();
                    ListBackgroundEntries(backgrounds.Root, sb, true);
                    return response.RespondString(sb.ToString());
                }
                else
                {
                    bool preview = false;
                    if (query.EndsWith("/preview"))
                    {
                        preview = true;
                        query = query.Substring(0, query.Length - "/preview".Length);
                    }

                    try
                    {
                        var file = backgrounds.GetFile(query);
                        var contentType = file.MimeType;

                        if (!String.IsNullOrEmpty(contentType))
                        {
                            response.SetHeader("Content-Type", contentType);
                        }
                        return response.RespondDownloaded(preview ? file.PreviewUri : file.Uri);
                    }
                    catch (FileNotFoundException)
                    {
                        return response.RespondNotFound();
                    }
                }
            }
            else
            {
                return next(env);
            }
        }