My uploaded files on Dropbox are 0 bytes/ empty.

this is my code :

public class UploadTask extends AsyncTask {
    private DbxClientV2 dbxClient;
    private File file;
    @SuppressLint("StaticFieldLeak")
    private Context context;
    ProgressDialog dialog;

    UploadTask(DbxClientV2 dbxClient, File file, Context context) {
        this.dbxClient = dbxClient;
        this.file = file;
        this.context = context;
    }

    @Override
    protected Object doInBackground(Object[] params) {
        try {
            // Log.e("async read",ReadFile(file));
            InputStream inputStream = new FileInputStream(file);
            dbxClient.files().uploadBuilder("/" +file.getName()) //Path in the user's Dropbox to save the file.
                    .withMode(WriteMode.OVERWRITE) //always overwrite existing file
                    .uploadAndFinish(inputStream);
            Log.e("Upoad", "" + dbxClient.files().uploadBuilder("/" + file.getName()) //Path in the user's Dropbox to save the file.
                    .withMode(WriteMode.OVERWRITE) //always overwrite existing file
                    .uploadAndFinish(inputStream));
        } catch (DbxException e) {
            e.printStackTrace();

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

    }

    @Override
    protected void onPostExecute(Object o) {
        super.onPostExecute(o);
        DownloadActivity.dialog.dismiss();
        Toast.makeText(context, "File uploaded successfully", Toast.LENGTH_SHORT).show();
        Intent intent = new Intent(context, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        context.startActivity(intent);

    }

    public static  String ReadFile(File file){
        String line = null;

        try {
            FileInputStream fileInputStream = new FileInputStream (file);
            InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
            BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
            StringBuilder stringBuilder = new StringBuilder();

            while ( (line = bufferedReader.readLine()) != null )
            {
                stringBuilder.append(line + System.getProperty("line.separator"));
            }
            fileInputStream.close();
            line = stringBuilder.toString();

            bufferedReader.close();
        }
        catch(FileNotFoundException ex) {
            Log.d("FileNotFoundException", ex.getMessage());
        }
        catch(IOException ex) {
            Log.d("IOException", ex.getMessage());
        }
        return line;
    }
}