AWS Lambda从S3上传文件至API遇422错误的解决方法

我有一个由指定S3存储桶中对象上传触发的AWS Lambda函数，预期在文件上传后发送携带该文件的POST请求。以下是初始代码：

import json
import os.path
import boto3
import requests

s3Client=boto3.client('s3')

def lambda_handler(event, context):
    bucket=event['Records'][0]['s3']['bucket']['name']
    key=event['Records'][0]['s3']['object']['key']
            
    FILE_NAME = os.path.basename(key)
    TMP_FILE_NAME = '/tmp/' +FILE_NAME
    s3Client.download_file(bucket, key,TMP_FILE_NAME)
    
    url = "*****************"

    payload={'approved': 'true',
            '': ''}
    files=[
      ('file',('ori-csv.csv',open(TMP_FILE_NAME,'rb'),'text/csv'))
      ]
    headers = {
      'Accept': 'application/json,application/vnd.api+json;version=1',
      'Content-Type': 'multipart/form-data',#(tried this also 'application/json')
      'Host': '****************',
      'Authorization': 'Bearer *****************'
    }

    
    response = requests.request("POST", url, headers=headers, data=payload, files=files)
    
    print(response)

已正确将requests库导入Lambda，且通过同一API的GET请求测试正常，但在CloudWatch中收到422响应，请问正确的请求头或文件POST方式是什么？

更新：问题已解决 #

将boto3客户端改为资源后问题解决，以下是正确代码：

s = boto3.resource('s3')
token = 'Bearer ******************'

def lambda_handler(event, context):
  bucket=event['Records'][0]['s3']['bucket']['name']
  key=event['Records'][0]['s3']['object']['key']
    
  print(bucket)
  print(key)
  file = key.split('/')[-1]
  print(file)
  s3_object = s.Object(bucket, key)
  data = s3_object.get()['Body'].read().decode('utf-8')

  with open('/tmp/'+file,'w') as file1:
    file1.write(data)
                  
  url = "**************"
              
  payload = {'approved': 'true'}
  files = [
        ('file',open('/tmp/'+file,'rb') )
          ]
  headers = {
          'Authorization': token
        }
    
  response = requests.request("POST", url, headers=headers, data=payload, files=files)

  print(response)

内容的提问来源于stack exchange，提问作者melamed