import requests
files = {'attachment_file': ('1.png', open('1.png', 'rb'), 'image/png', {})}
values = {'next':"http://www.xxxx.com/xxxx"}
r = requests.post('http://www.xxxx.com/upload', files=files, data=values) # 成功
r = requests.post('http://www.xxxx.com/upload', files=files, data=values) # 失败
r = requests.post('http://www.xxxx.com/upload', files=files, data=values) # 失败
r = requests.post('http://www.xxxx.com/upload', files=files, data=values) # 失败
r = requests.post('http://www.xxxx.com/upload', files=files, data=values) # 失败
...

......
form = AttachmentForm(request.POST, request.FILES)
if form.is_valid():
 form.save(request, obj)
 messages.success(request,_('Your attachment was uploaded.'))
 return HttpResponseRedirect(next)
......

def to_python(self, data):
 if data in validators.EMPTY_VALUES:
 return None
 # UploadedFile objects should have name and size attributes.
 try:
 file_name = data.name
 file_size = data.size
 except AttributeError:
 raise ValidationError(self.error_messages['invalid'])
 if self.max_length is not None and len(file_name) > self.max_length:
 error_values = {'max': self.max_length, 'length': len(file_name)}
 raise ValidationError(self.error_messages['max_length'] % error_values)
 if not file_name:
 raise ValidationError(self.error_messages['invalid'])
 if not self.allow_empty_file and not file_size:
 raise ValidationError(self.error_messages['empty'])
 return data

#正常情况
In [28]: r = requests.post('http://www.xxxx.com/upload', files=files, data=values)
In [29]: r.request.body
#错误情况
In [33]: r = requests.post('http://www.xxxx.com/upload', files=files, data=values)
In [34]: r.request.body
Out[34]: '--155322d3e780432bb06e58135e041c8f\r\nContent-Disposition: form-data; name="next"\r\n\r\nhttp://www.xxxx.com/upload\r\n--155322d3e780432bb06e58135e041c8f\r\nContent-Disposition: form-data; name="attachment_file"; filename="1.png"\r\nContent-Type: image/png\r\n\r\n\r\n--155322d3e780432bb06e58135e041c8f--\r\n'

fl = open('1.png','rb')
files = {'attachment_file': ('1.png', fl, 'image/png', {})}
r1 = requests.post('http://www.xxxx.com/upload', files=files, data=values)
fl.close()
fl = open('1.png','rb')
files = {'attachment_file': ('1.png', fl, 'image/png', {})}
r2 = requests.post('http://www.xxxx.com/upload', files=files, data=values)