HTTP-Post with Requests fails: BadStatusLine

Question

I got a problem with the following code. While trying to upload a file to the API using a POST-Request I get an error saying

File "upload.py", line 44, in <module>
    so.uploadData("./test.txt")
  File "upload.py", line 37, in uploadData
    req = requests.post("http://"+server, data=payload, headers=headers)
  File "/Library/Python/2.7/site-packages/requests/api.py", line 109, in post
    return request('post', url, data=data, json=json, **kwargs)
  File "/Library/Python/2.7/site-packages/requests/api.py", line 50, in request
    response = session.request(method=method, url=url, **kwargs)
  File "/Library/Python/2.7/site-packages/requests/sessions.py", line 468, in request
    resp = self.send(prep, **send_kwargs)
  File "/Library/Python/2.7/site-packages/requests/sessions.py", line 576, in send
    r = adapter.send(request, **kwargs)
  File "/Library/Python/2.7/site-packages/requests/adapters.py", line 412, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', BadStatusLine("''",))

My Code looks like this:

filedata= open(path, 'r')
payload = {'name': self.username, 'file':filedata}
headers = {
  'User-Agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.110 Safari/537.36',
  'content-type': "multipart/form-data"}
req = requests.post("http://"+server, data=payload, headers=headers)

Does anyone have a solution for this problem? Best regards