android uploading a file to a nodejs server

Question

I'm trying to upload a file from my android device to a nodejs server, I'm using okhttp to send a post request.The request reached the server but I can't access the file in the request to save it..I tried to put the data i provided in the header but they are all gives undefined..... here is the code:

MainActivity:

    package app.file_upload;

import android.Manifest;
import android.app.ProgressDialog;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.webkit.MimeTypeMap;
import android.widget.Button;

import com.nbsp.materialfilepicker.MaterialFilePicker;
import com.nbsp.materialfilepicker.ui.FilePickerActivity;

import java.io.File;
import java.io.IOException;

import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {

    private Button button;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button) findViewById(R.id.button);


        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M){
            if(ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED){
                requestPermissions(new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},100);
                return;
            }
        }

        enable_button();
    }

    private void enable_button() {

            button.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    new MaterialFilePicker()
                            .withActivity(MainActivity.this)
                            .withRequestCode(10)
                            .start();

                }
            });
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        if(requestCode == 100 && (grantResults[0] == PackageManager.PERMISSION_GRANTED)){
            enable_button();
        }else {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                requestPermissions(new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},100);
            }
        }
    }

    ProgressDialog progress;

    @Override
    protected void onActivityResult(int requestCode, int resultCode, final Intent data) {
        if(requestCode == 10 && resultCode == RESULT_OK){

            progress = new ProgressDialog(MainActivity.this);
            progress.setTitle("Uploading");
            progress.setMessage("Please wait...");
            progress.show();

            Thread t = new Thread(new Runnable() {
                @Override
                public void run() {

                    File f  = new File(data.getStringExtra(FilePickerActivity.RESULT_FILE_PATH));
                    String content_type  = getMimeType(f.getPath());

                    String file_path = f.getAbsolutePath();
                    OkHttpClient client = new OkHttpClient();
                    RequestBody file_body = RequestBody.create(MediaType.parse(content_type),f);

                    RequestBody request_body = new MultipartBody.Builder()
                            .setType(MultipartBody.FORM)
                            .addFormDataPart("type",content_type)
                            .addFormDataPart("uploaded_file",file_path.substring(file_path.lastIndexOf("/")+1), file_body)
                            .build();

                    Request request = new Request.Builder()
                            .url("http://192.168.0.109:2999/upload_audio")
                            .post(request_body)
                            .build();

                    try {
                        Response response = client.newCall(request).execute();

                        if(!response.isSuccessful()){
                            throw new IOException("Error : "+response);
                        }

                        progress.dismiss();

                    } catch (IOException e) {
                        e.printStackTrace();
                    }


                }
            });

            t.start();




        }
    }

    private String getMimeType(String path) {

        String extension = MimeTypeMap.getFileExtensionFromUrl(path);

        return MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
    }
}

Now to the nodeJs code: //after initiating the server ...

  app.post('/upload_audio', function(req, res) {
  if (!req.files)
    return res.status(400).send('No files were uploaded.');

  // The name of the input field (i.e. "sampleFile") is used to retrieve the uploaded file
  let sampleFile = req.files.sampleFile;

  // Use the mv() method to place the file somewhere on your server
  sampleFile.mv('/somewhere/on/your/server/filename.jpg', function(err) {
    if (err)
      return res.status(500).send(err);

    res.send('File uploaded!');
  });
});