diff --git a/src/pretix/helpers/http.py b/src/pretix/helpers/http.py
index fd32d8bd4f..d1471080a8 100644
--- a/src/pretix/helpers/http.py
+++ b/src/pretix/helpers/http.py
@@ -5,5 +5,7 @@ class ChunkBasedFileResponse(StreamingHttpResponse):
     block_size = 4096
 
     def __init__(self, streaming_content=(), *args, **kwargs):
+        filelike = streaming_content
         streaming_content = streaming_content.chunks(self.block_size)
         super().__init__(streaming_content, *args, **kwargs)
+        self['Content-Length'] = filelike.size