added file saver

lib/utils/FileSaver.dart

@@ -0,0 +1,23 @@
+import 'dart:io';
+
+import 'package:permission_handler/permission_handler.dart';
+
+// only tested on android!
+class FileSaver {
+  static Future<String> getExternalDocumentPath() async {
+    var permission = await Permission.storage.status;
+    if(!permission.isGranted) {
+      await Permission.storage.request();
+    }
+    var directory = Directory('/storage/emulated/0/Download');
+    final externalPath = directory.path;
+    await Directory(externalPath).create(recursive: true);
+    return externalPath;
+  }
+
+  static Future<File> writeBytes(List<int> bytes, String name) async {
+    final path = await getExternalDocumentPath();
+    var file = File('$path/$name');
+    return file.writeAsBytes(bytes);
+  }
+}