为什么mime读取未知扩展名文件时，png文件是可以获取mime，但是mp4格式文件或者mime是null

`mime`包是通过文件扩展名来判断MIME类型的，如果文件扩展名不在其内置的列表中，则会返回`null`。对于未知扩展名的文件，您可以尝试使用第三方库来获取MIME类型，例如`file_picker`包中的`FileType`类可以获取许多常见文件类型的MIME类型。 以下是一个示例代码： ```dart import 'dart:io'; import 'package:flutter/material.dart'; import 'package:path_provider/path_provider.dart'; import 'package:permission_handler/permission_handler.dart'; import 'package:file_picker/file_picker.dart'; void main() { runApp(MyApp()); } class MyApp extends StatelessWidget { @override Widget build(BuildContext context) { return MaterialApp( title: 'Flutter Demo', home: MyHomePage(), ); } } class MyHomePage extends StatefulWidget { @override _MyHomePageState createState() => _MyHomePageState(); } class _MyHomePageState extends State { String _filePath; String _mimeType; @override void initState() { super.initState(); _requestPermission(); } Future _requestPermission() async { if (await Permission.storage.request().isGranted) { // Permission is granted, continue with the app _getFile(); } else { // Permission is not granted, show an error message showDialog( context: context, builder: (context) => AlertDialog( title: Text('Error'), content: Text('Please grant storage permission to use this app.'), actions: [ FlatButton( child: Text('OK'), onPressed: () => Navigator.pop(context), ), ], ), ); } } Future _getFile() async { File file = await FilePicker.getFile(type: FileType.any); if (file != null) { setState(() { _filePath = file.path; _mimeType = lookupMimeType(_filePath); }); } else { showDialog( context: context, builder: (context) => AlertDialog( title: Text('Error'), content: Text('File not found.'), actions: [ FlatButton( child: Text('OK'), onPressed: () => Navigator.pop(context),