fix: provide a file URL by using url method

lms/djangoapps/mobile_api/course_info/views.py

@@ -425,10 +425,9 @@ def _extend_block_info_with_offline_data(blocks_info_data: Dict[str, Dict]) -> N
         """
         for block_id, block_info in blocks_info_data.items():
             if offline_content_path := get_offline_block_content_path(usage_key=UsageKey.from_string(block_id)):
-                file_url = os.path.join(settings.MEDIA_URL, offline_content_path)
                 block_info.update({
                     'offline_download': {
-                        'file_url': file_url,
+                        'file_url': default_storage.url(offline_content_path),
                         'last_modified': default_storage.get_modified_time(offline_content_path),
                         'file_size': default_storage.size(offline_content_path)
                     }