我在React Native上有一个前端，并且实现了以下内容：

import React,{Component} from 'react';
import Config from 'react-native-config';
import {
  Button,Alert,Image,Text,View,StyleSheet,TouchableOpacity,Platform,CameraRoll,} from 'react-native';
import {Container,Content,Icon} from 'native-base';
import {RNCamera} from 'react-native-camera';
import {SubHeader} from '../../components/headers';

const styles = StyleSheet.create({
  container: {
    flex: 1,flexDirection: 'column',backgroundColor: 'black',},preview: {
    flex: 1,justifyContent: 'flex-end',alignItems: 'center',capture: {
    flex: 0,backgroundColor: '#fff',borderRadius: 5,padding: 15,paddingHorizontal: 20,alignSelf: 'center',margin: 20,});

export default class MainScreen extends Component {
  constructor(props) {
    super(props);
    this.state = {
      imageUri: null,};
  }

  test = async () => {
    if (this.camera) {
      const data = await this.camera.takePictureAsync({
        quality: 0.5,base64: true,});

      const photo = {
        name: data.fileName,type: data.type,uri:
          Platform.OS === 'android'
            ? data.uri
            : data.uri.replace('file://',''),};

      this.setState({imageUri: data.uri}); //preview
      const fdata = new FormData();
      fdata.append('file',photo);

      try {
        let response = await fetch(`${Config.BASE_URL}/scan`,{
          method: 'POST',body: JSON.stringify(fdata),//body: fdata //this launches a connection error
        });
        const res = await response.json();

        return res;
      } catch (error) {
        console.error(error);
      }
    } else {
      Alert.alert('debug','There is no camera');
    }
  };

  render() {
    const {navigate} = this.props.navigation;

    return (
          <View style={styles.container}>
            <RNCamera
              ref={ref => {
                this.camera = ref;
              }}
              style={styles.preview}
              type={RNCamera.Constants.Type.back}
              flashMode={RNCamera.Constants.flashMode.off}
              captureAudio={false}
              androidCameraPermissionOptions={{
                title: 'Permission to use camera',message: 'We need your permission to use your camera',buttonPositive: 'Ok',buttonNegative: 'Cancel',}}
              onGoogleVisionBarcodesDetected={({barcodes}) => {
                console.log(barcodes);
              }}
            />
            <View>
              <TouchableOpacity
                onPress={this.test.bind(this)}
                style={styles.capture}>
                <Icon type="FontAwesome" ios="camera" android="camera" />
              </TouchableOpacity>
            </View>
          </View>

           //preview
          <Image
            style={{width: 66,height: 58}}
            source={{
              uri: this.state.imageUri,}}
          />
        </View>
    );
  }
}

在具有nodeJs表达和multer的后端：

app.post('/scan',uploader.upload.single('file'),ScanController.scan);

Multer的实现很好，因为它与邮递员和前端Web应用程序功能一起工作。

该图像显示在android设备上，但是后端总是出现一个未定义的对象，我不知道如何发送该图像，因为它基于base 64，如何正确发送或接收该图像？