Requests and Requests_Toolbelt filesize limit

Using request_toolbelt's MultipartEncoder, I am able to upload files to my Pyramid server, but only up to a certain size. Once the file are beyond a certain size (not sure of the exact size), requests throws an ConnectionError. My Pyramid server shows absolutely no sign of ever getting the request. The callback monitor shows 2 or 3 chunks read but then it aborts. Why?? Here is my code:

### client-side

# callback
def cb(monitor):
    print monitor.bytes_read

file = open('my_big_file.mpg', 'rb')
payload = MultipartEncoder({'uploadedFile': (file.name, file, 'application/octet-stream')})
monitor = MultipartEncoderMonitor(payload, cb)
r = requests.post(url, data=monitor, headers={'Content-Type': payload.content_type})

#### server-side

@view_config(route_name='remote.agent_upload', renderer='json')
def remote_agent_upload(request):
r = request.response
uploadedFile = request.POST['uploadedFile']
fs = uploadedFile.file
filename = uploadedFile.filename
f = open('path_to_storage' + filename, 'wb')
f.write(fs.read())
fs.close()
f.close()
return r

### output and traceback

8192
16384
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\Python27\lib\site-packages\requests\api.py", line 109, in post
    return request('post', url, data=data, json=json, **kwargs)
  File "c:\Python27\lib\site-packages\requests\api.py", line 50, in request
    response = session.request(method=method, url=url, **kwargs)
  File "c:\Python27\lib\site-packages\requests\sessions.py", line 465, in request
    resp = self.send(prep, **send_kwargs)
  File "c:\Python27\lib\site-packages\requests\sessions.py", line 573, in send
    r = adapter.send(request, **kwargs)
  File "c:\Python27\lib\site-packages\requests\adapters.py", line 415, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', error(10053, 'An established connection was aborted by the software in your host machine'))

Solution

Windows is closing the connection for an unspecified reason. See this other answer for more detail.

In short, this is neither the fault of requests, requests-toolbelt, or pyramid.