我的python代码如下：

request_url = "https://aip.baidubce.com/api/v1/solution/direct/img_censor"
# 二进制方式打开图片文件
f = open(imgfile, 'rb')
img = base64.b64encode(f.read())
params = {"image":img}
params = urllib.urlencode(params)

#access_token = '[调用鉴权接口获取的token]'
request_url = request_url + "?access_token=" + access_token
request = urllib2.Request(url=request_url, data=params)
request.add_header('Content-Type', 'application/x-www-form-urlencoded')
response = urllib2.urlopen(request)
content = response.read()
if content:
    print content

报错如下：

Traceback (most recent call last):
  File "D:/python/test/baiduAI.py", line 53, in 
    response = urllib2.urlopen(request)
  File "C:\Python27\lib\urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "C:\Python27\lib\urllib2.py", line 437, in open
    response = meth(req, response)
  File "C:\Python27\lib\urllib2.py", line 550, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\Python27\lib\urllib2.py", line 475, in error
    return self._call_chain(*args)
  File "C:\Python27\lib\urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "C:\Python27\lib\urllib2.py", line 558, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 415: Unsupported Media Type

调用色情和恐暴接口都没问题，调用政治敏感和恶心图像就不行，请问各位大神这是因为什么？