Results in:
```
AssertionError: null
File "synapse/http/server.py", line 332, in _async_render_wrapper
callback_return = await self._async_render(request)
File "synapse/http/server.py", line 544, in _async_render
callback_return = await raw_callback_return
File "synapse/federation/transport/server/_base.py", line 369, in new_func
response = await func(
File "synapse/federation/transport/server/federation.py", line 826, in on_GET
await self.media_repo.get_local_media(
File "synapse/media/media_repository.py", line 473, in get_local_media
await respond_with_multipart_responder(
File "synapse/media/_base.py", line 353, in respond_with_multipart_responder
assert content_length is not None
```
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