UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte

Salesforceの添付データを取得し、新規フォルダを作成ご保存数するプログラムを作っているのですが、下記のエラーを解決できず詰まっています。

File "autotmp.py", line 50, in <module>
resp = sfdc.get(urlpath).decode('utf-8')
File "/usr/local/lib/python3.5/dist-packages/salesforce/api.py", line 49, in get
return self.__get_api(soap).get(get_url, params)
File "/home/dbcon/utils.py", line 188, in authenticate_and_call
return func(self, *args, **kwargs)
File "/home/dbcon/salesforceRestApi.py", line 104, in get
params=params)
File "/home/dbcon/salesforceRestApi.py", line 137, in __send_request
**kwargs)
File "/home/dbcon/utils.py", line 159, in send_request
resp = response.json()
File "/usr/lib/python3/dist-packages/requests/models.py", line 740, in json
return json.loads(self.content.decode(encoding), **kwargs)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte

どなたか解決策を教えていただけないでしょうか。
宜しくお願いします。

python3.5

#!/usr/bin/env python
# -*- coding: utf-8 -*-
#Automatically download attached files from records
#To download multiple files as zip use:
#  https://fsi-america-dev-ed.my.salesforce.com/sfc/servlet.shepherd/document/download/<id>/<id>/...

import salesforce as sf
import os
import urllib.request

sfdc = sf.Salesforce()
sfdc.authenticate(client_id=client_id,client_secret=client_secret,username=username,password=password)

#get api sobjects
api_objects = []
urlpath = "/sobjects/"
resp = sfdc.get(urlpath,'utf-8')
for record in resp["sobjects"]:
  api_objects.append(record["name"])

#get id, filename from contentversion
version = {}
query_results = sfdc.query("SELECT Id, ContentDocumentId, Title, FileExtension FROM ContentVersion")
for record in query_results["records"]:
  if record["ContentDocumentId"] not in version:
    version[record["ContentDocumentId"]] = []
  version[record["ContentDocumentId"]].append({"Id":record["Id"],"Filename":record["Title"] + "." + record["FileExtension"]})

#download
for api_object in api_objects:
  print("processing:" + api_object, 'utf-8')
  query_results = None
  try:
    query_results = sfdc.query("SELECT Id, ContentDocumentId, LinkedEntityId FROM ContentDocumentLink where LinkedEntityId in (SELECT Id FROM " + api_object + ")")
  except:
    continue
  for record in query_results["records"]:
    if record["ContentDocumentId"] in version:
      vlist = version[record["ContentDocumentId"]]
      for v in vlist:
        print("  downloading:" + v["Id"], 'utf-8')
        urlpath = "/sobjects/ContentVersion/" + v["Id"] + "/VersionData"
        filepath = base_folder + api_object + "/" + record["LinkedEntityId"] + "/" + v["Filename"]
        resp = sfdc.get(urlpath, 'utf-8')
        os.makedirs(os.path.dirname(filepath), exist_ok=True)
        with open(filepath, "wb", encoding='utf-8') as f:
          f.write(resp.content)