CacheManager attempts to create file named guid.* on Windows (full example included and clear description)

疑似逻辑位于 lib/src/web/file_service.dart 第 115 行附近；首先在 Windows 上运行提供的最小复现，并跟踪 content-type 标头如何变成缓存文件名的扩展名。完成的标准是：对于标头未提供可用扩展名的下载，能够创建符合 Windows 要求的文件名，同时普通扩展名仍能正常工作。

🐛 Bug Report

While downloading an image from a webserver the cache manager decides to create a filename ending with .* which is not permitted on windows.

Expected behavior

Create a filename without extension or a generic extension like .cache or .file since the cache manager cannot determine it from the webserver headers.

Reproduction steps

i have made minimal reproducible example no external dependencies needed (other than http package)
i am aware that the server should provide correct descriptive headers for the content being served but sometimes it happens not to in some scenarios.

import 'package:flutter_cache_manager/flutter_cache_manager.dart';
import 'package:http/http.dart' as http;
import 'package:http/testing.dart';

Future<void> main() async {
  final client = MockClient((http.Request request) async =>
      http.Response("a png image", 200, headers: {
        "content-type": "application/*",
        "content-disposition":
            "attachment; filename=file; filename*=UTF-8''file"
      }));

  final cmgr = CacheManager(Config(DefaultCacheManager.key,
      fileService: HttpFileService(httpClient: client)));

/* crashes with
  Launching lib\main.dart on Windows in debug mode...
  √  Built build\windows\runner\Debug\cache_mgr_test.exe.
  [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: FileSystemException: Cannot open file, path = 'C:\Users\csman\AppData\Local\Temp\libCachedImageData\cf09f070-6cef-1d8a-b096-2334712b3134.*' (OS Error: The filename, directory name, or volume label syntax is incorrect.

  , errno = 123)
  #0      _checkForErrorResponse (dart:io/common.dart:55:9)
  #1      _File.open.<anonymous closure> (dart:io/file_impl.dart:381:7)
  <asynchronous suspension>
*/

  await cmgr.downloadFile("http://myaddress.com/imageManager?pngfileid=1234");
}

/*
pubspec.yaml 

name: cache_mgr_test
description: A new Flutter project.
publish_to: "none"
version: 1.0.0+1
environment:
  sdk: ">=3.1.5 <4.0.0"
dependencies:
  cupertino_icons: ^1.0.2
  flutter:
    sdk: flutter
  flutter_cache_manager: ^3.3.1
  http: ^1.1.0
dev_dependencies:
  flutter_lints: ^2.0.0
  flutter_test:
    sdk: flutter
flutter:
  uses-material-design: true
*/

Configuration

Version: 3.3.1

Platform:
Windows 10

I think the problematic part is in \flutter_cache_manager-3.3.1\lib\src\web\file_service.dart line 115 around

  @override
  String get fileExtension {
    var fileExtension = '';
    final contentTypeHeader = _header(HttpHeaders.contentTypeHeader);
    if (contentTypeHeader != null) {
      final contentType = ContentType.parse(contentTypeHeader);
      fileExtension = contentType.fileExtension;
    }
    return fileExtension;
  }