Media upload fails with `ValueError: seek of closed file`

This issue has been migrated from [#6192](https://github.com/matrix-org/synapse/issues/6192).

---

### Description

Media uploads are failing.

### Steps to reproduce

Uploading any file in Riot (any platform) results in the upload failing.

```
2019-10-10 14:57:47,521 - synapse.http.server - 109 - ERROR - POST-329537- Failed handle request via 'UploadResource':
Traceback (most recent call last):
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/http/server.py", line 77, in wrapped_request_handler
await h(self, request)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/rest/media/v1/upload_resource.py", line 82, in _async_render_POST
media_type, upload_name, request.content, content_length, requester.user
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
result = result.throwExceptionIntoGenerator(g)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/failure.py", line 512, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/rest/media/v1/media_repository.py", line 160, in create_content
fname = yield self.media_storage.store_file(content, file_info)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
result = result.throwExceptionIntoGenerator(g)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/failure.py", line 512, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/rest/media/v1/media_storage.py", line 68, in store_file
self.hs.get_reactor(), _write_file_synchronously, source, f
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/threadpool.py", line 250, in inContext
result = inContext.theWork()
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/threadpool.py", line 266, in
inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/context.py", line 122, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/python/context.py", line 85, in callWithContext
return func(*args,**kw)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/logging/context.py", line 716, in g
return f(*args, **kwargs)
File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/rest/media/v1/media_storage.py", line 244, in _write_file_synchronously
source.seek(0) # Ensure we read from the start of the file
ValueError: seek of closed file
2019-10-10 14:57:47,578 - synapse.http.server - 458 - WARNING - POST-329537- Not sending response to request , already disconnected.
```

I also see similar errors ending in `ValueError: I/O operation on closed file.` on some uploads. I can upload the homeserver.log if needed.

### Version information

- **Homeserver**: chat.privacytools.io

If not matrix.org:

- **Version**: 1.4.0

- **Install method**: apt

- **Platform**: Ubuntu 18.04.3 LTS, running in an LXC container. The Matrix HS is behind an nginx reverse proxy on another container/