SalesforceMalformedRequest: Malformed request
https://my_org.my.salesforce.com/services/data/v38.0/sobjects/Attachment/.
Response content: [{
   'message': "Json Deserialization failed on token 'null' and has left off in the middle of parsing a row. Will go to end of row to begin parsing the next row",
   'errorCode': 'INVALID_FIELD'}]

from simple_salesforce import Salesforce
from base64 import b64encode
import json

userName = 'my_username@test.com'
password = 'mypw'
securityToken = 'mytoken'
svc = Salesforce(username=userName, password=password, security_token=securityToken)
ENCODING = 'utf-8'
name = 'test.txt'
parent_id = '5001I000006rNenQAE'


with open(name, 'rb') as open_file:
    byte_content = open_file.read()
    base64_bytes = b64encode(byte_content)
    base64_string = base64_bytes.decode(ENCODING)
    json_data = json.dumps({
                    'ParentId' : parent_id,
                    'Name' : name,
                    'Body' : base64_string
                    })

    result = svc.Attachment.create(json_data)
    print(result)

svc.Attachment.create({ 'ParentId': parent_id, 'Name': name, 'Body': base64_string })