在用于拾取图像的方法中，只需修改局部变量imageData并不返回任何内容。

Future pickImage({File? imageData, html.File? htmlImageData}) async {
    final XFile? image =
      await _picker.pickImage(source: ImageSource.camera, imageQuality: 50);
    if (image != null) {
      setState(() {
        imageData = File(image.path); // Won't do anything since variable is in local scope
      });
    }
    if (kDebugMode) {
      print(serviceData.fileBukuServis!.path);
    }
  }

您可以直接更改serviceData.fileBukuServis变量。下面是修改后的代码：

class ServiceData {
   File? fileBukuServis;
   File? htmlfileBukuServis;
   File? fileSTNK;
   File? fileOwnerManual;
}

enum ServiceDataFile{
   fileBukuServis,
   htmlfileBukuServis,
}

class _DetailCarState extends State<DetailCar> {
  ServiceData serviceData = ServiceData();
  final ImagePicker _picker = ImagePicker();

  Future pickImage(ServiceDataFile fileMode) async {
    final XFile? image =
        await _picker.pickImage(source: ImageSource.camera, imageQuality: 50);
    if (image != null) {
      if(fileMode == ServiceDataFile.fileBukuServis){
         serviceData.fileBukuServis = File(image.path);
      } else {
         serviceData.htmlfileBukuServis = File(image.path);
      }
      setState((){});

      if(kDebugMode){
         print(image.path);
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      style: ButtonStyle(
        backgroundColor:
            MaterialStateProperty.all(Colors.red[800]),
      ),
      onPressed: () {
        pickImage(kIsWeb ? ServiceDataFile.htmlfileBukuServis : ServiceDataFile.fileBukuServis);
      },
      child: const Text('Ambil gambar'),
    );
  }
}

另外，您的ServiceData类没有htmlfileBukuServis字段。确保将其添加到代码中。