Object Storage Service:How do I restrict file types and size when I upload objects to OSS?

import os
from hashlib import sha1 as sha
import json
import base64
import hmac
import datetime
import time

# Configure the OSS_ACCESS_KEY_ID environment variable. 
access_key_id = os.environ.get('OSS_ACCESS_KEY_ID')
# Configure the OSS_ACCESS_KEY_SECRET environment variable. 
access_key_secret = os.environ.get('OSS_ACCESS_KEY_SECRET')
# Replace <your-bucket> with the name of the bucket. 
bucket = '<your-bucket>'
# Set host to a value that is in the bucketname.endpoint format. Replace <your-bucket> with the name of the bucket. Replace <your-endpoint> with the OSS endpoint. Example: oss-cn-hangzhou.aliyuncs.com. 
host = 'https://<your-bucket>.<your-endpoint>'
# Specify the prefix of the object that you want to upload to OSS. 
upload_dir = 'user-dir-prefix/'
# Specify the validity period of the signature and the policy. Unit: seconds. 
expire_time = 3600


def generate_expiration(seconds):
    """
    The expiration time is calculated based on the specified validity period. Unit: seconds. 
    :param seconds: validity period (seconds). 
    :return: time string in the ISO 8601 standard. Example: 2014-12-01T12:00:00.000Z. 
    """
    now = int(time.time())
    expiration_time = now + seconds
    gmt = datetime.datetime.utcfromtimestamp(expiration_time).isoformat()
    gmt += 'Z'
    return gmt


def generate_signature(access_key_secret, expiration, conditions, policy_extra_props=None):
    """
    Generate a signature string. 
    :param access_key_secret: the AccessKey secret that has the permissions to access the bucket. 
    :param expiration: the time when the signature expires. Specify the time in the ISO 8601 standard in the yyyy-MM-ddTHH:mm:ssZ format. Example: 2014-12-01T12:00:00.000Z. 
    :param conditions: the policy conditions that can be used to limit the values that you can set during form upload. 
    :param policy_extra_props: the additional policy parameters. If additional parameters are supported by the policy, you can pass additional parameters by using dict. 
    :return: the signature string. 
    """
    policy_dict = {
        'expiration': expiration,
        'conditions': conditions
    }
    if policy_extra_props is not None:
        policy_dict.update(policy_extra_props)
    policy = json.dumps(policy_dict).strip()
    policy_encode = base64.b64encode(policy.encode())
    h = hmac.new(access_key_secret.encode(), policy_encode, sha)
    sign_result = base64.b64encode(h.digest()).strip()
    return sign_result.decode()

def generate_upload_params():
    policy = {
        # The validity period of the policy. 
        "expiration": generate_expiration(expire_time),
        # Specify the conditions in the policy field. 
        "conditions": [
            # If success_action_redirect is not specified, the HTTP status code returned after a successful upload is 204. 
            ["eq", "$success_action_status", "200"],
            # The value of the form field must start with the specified prefix. For example, if the value of the key form field starts with user/user1, the condition is ["starts-with", "$key", "user/user1"]. 
            ["starts-with", "$key", upload_dir],
            # Specify the minimum and maximum sizes of the object that you want to upload. Unit: bytes. 
            ["content-length-range", 1, 1000000],
            # Set the type of the object that you want to upload to the specified image type.
            ["in", "$content-type", ["image/jpg", "image/png"]]
        ]
    }
    signature = generate_signature(access_key_secret, policy.get('expiration'), policy.get('conditions'))
    response = {
        'policy': base64.b64encode(json.dumps(policy).encode('utf-8')).decode(),
        'ossAccessKeyId': access_key_id,
        'signature': signature,
        'host': host,
        'dir': upload_dir
        # Specify additional parameters.
    }
    return json.dumps(response)

const form = document.querySelector('form');
const fileInput = document.querySelector('#file');
form.addEventListener('submit', (event) => {
  event.preventDefault();
  let file = fileInput.files[0];
  let filename = fileInput.files[0].name;
  fetch('/get_post_signature_for_oss_upload', { method: 'GET' })
    .then(response => response.json())
    .then(data => {
      const formData = new FormData();
      formData.append('name',filename);
      formData.append('policy', data.policy);
      formData.append('OSSAccessKeyId', data.ossAccessKeyId);
      formData.append('success_action_status', '200');
      formData.append('signature', data.signature);
      formData.append('key', data.dir + filename);
      // file must be the last form field. No order is required for other form fields. 
      formData.append('file', file);
      fetch(data.host, { method: 'POST', body: formData },).then((res) => {
        console.log(res);
        alert ('Object Uploaded');
      });
    })
    .catch(error => {
      console.log('Error occurred while getting OSS upload parameters:', error);
    });
});

import oss2
from oss2.credentials import EnvironmentVariableCredentialsProvider

# Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. 
auth = oss2.ProviderAuth(EnvironmentVariableCredentialsProvider())
# Replace <your-endpoint> with the endpoint of the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com. 
# Replace <your-bucket> with the name of the bucket. 
bucket = oss2.Bucket(auth, '<your-endpoint>', '<your-bucket>')
# Specify the validity period. Unit: seconds. 
expire_time = 3600
# Specify the full path of the object. Example: exampledir/exampleobject.png. Do not include the bucket name in the full path. 
object_name = 'exampledir/exampleobject.png'

def generate_presigned_url():
    # Specify headers. 
    headers = dict()
    # Specify the Content-Type header. 
    headers['Content-Type'] = 'image/png'
    # Specify the storage class of the object. 
    # headers["x-oss-storage-class"] = "Standard"
    # By default, OSS identifies the forward slashes (/) in the full path of an object as escape characters when a signed URL is generated. Therefore, the signed URL cannot be used directly. 
    # Set the slash_safe parameter to True. This way, OSS does not identify the forward slashes (/) in the full path of the object as escape characters. Then, you can use the generated signed URL to upload the object. 
    url = bucket.sign_url('PUT', object_name, expire_time, slash_safe=True, headers=headers)
    return url

const form = document.querySelector("form");
form.addEventListener("submit", (event) => {
  event.preventDefault();
  const fileInput = document.querySelector("#file");
  const file = fileInput.files[0];
  fetch(`/get_presigned_url_for_oss_upload?filename=${file.name}`, { method: "GET" })
    .then((response) => {
        return response.text();
     })
    .then((url) => {
      fetch(url, {
        method: "PUT",
        headers: new Headers({
          'Content-Type': 'image/png',
        }),
        body: file,
       }).then((res) => {
            console.log(res);
            alert ('Object Uploaded');
       });
   });
});