Work with cbz files with subdirectories

backend.py

@@ -260,7 +260,7 @@ class CalibreDB(BaseDB):
         with ZipFile(path, 'r') as volume:
             try:
                 # filter sub directories
-                filelist = [file for file in f.filelist if not file.is_dir()]
+                filelist = [file for file in volume.filelist if not file.is_dir()]
                 zip_info = filelist[page_number - 1]
             except IndexError:
                 raise exceptions.NotFound()