liuniu/vendor/qiniu/php-sdk/examples/rs_change_mime.php

<?php
require_once __DIR__ . '/../autoload.php';

use \Qiniu\Auth;

$accessKey = getenv('QINIU_ACCESS_KEY');
$secretKey = getenv('QINIU_SECRET_KEY');
$bucket = getenv('QINIU_TEST_BUCKET');

$key = 'qiniu.mp4';
$newMime = 'video/x-mp4';

$auth = new Auth($accessKey, $secretKey);
$config = new \Qiniu\Config();
$bucketManager = new \Qiniu\Storage\BucketManager($auth, $config);

$err = $bucketManager->changeMime($bucket, $key, $newMime);
if ($err) {
    print_r($err);
}