Added 'isZipFile' function

lib/flutter_archive.dart

@@ -214,3 +214,17 @@ class ZipEntry {
   final int? crc;
   final CompressionMethod? compressionMethod;
 }
+
+///
+/// Checks whether the file starts with the needed file header
+///
+Future<bool> isZipFile(File file) async {
+  // https://en.wikipedia.org/wiki/ZIP_(file_format)
+  final randomAccessFile = file.openSync(mode: FileMode.read);
+  final bytes = await randomAccessFile.read(4);
+  return bytes.length > 3 &&
+      bytes[0] == 0x50 &&
+      bytes[1] == 0x4B &&
+      bytes[2] == 0x03 &&
+      bytes[3] == 0x04;
+}

pubspec.yaml

@@ -11,10 +11,12 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
+
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  mockito: ^5.0.16
 
 # For information on the generic Dart part of this file, see the
 # following page: https://dart.dev/tools/pub/pubspec

test/flutter_archive_test.dart

@@ -1,10 +1,16 @@
+import 'dart:io';
+import 'dart:typed_data';
+
 import 'package:flutter/services.dart';
+import 'package:flutter_archive/flutter_archive.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:mockito/mockito.dart';
 
 void main() {
   const MethodChannel channel = MethodChannel('flutter_archive');
 
   setUp(() {
+    TestWidgetsFlutterBinding.ensureInitialized();
     channel.setMockMethodCallHandler((MethodCall methodCall) async {
       return true;
     });
@@ -17,4 +23,34 @@ void main() {
   test('getPlatformVersion', () async {
     // TODO
   });
+
+  test('isZipFile', () async {
+    expect(await isZipFile(MockFile(true)), true);
+    expect(await isZipFile(MockFile(false)), false);
+  });
+}
+
+class MockFile extends Mock implements File, RandomAccessFile {
+  final bool _isZipFile;
+
+  MockFile(this._isZipFile);
+
+  @override
+  Future<RandomAccessFile> open({FileMode mode = FileMode.read}) async {
+    return this;
+  }
+
+  @override
+  RandomAccessFile openSync({FileMode mode = FileMode.read}) {
+    return this;
+  }
+
+  @override
+  Future<Uint8List> read(int count) async {
+    if (_isZipFile) {
+      return Uint8List.fromList([0x50, 0x4B, 0x03, 0x04]);
+    } else {
+      return Uint8List.fromList([]);
+    }
+  }
 }