Wie kann ich mit Volley mehrteilige POST-Anfragen stellen, ohne HttpEntity zu verwenden?

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.widget.Toast;

import com.android.volley.NetworkResponse;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.example.multipartvolley.MultipartRequest;
import com.example.multipartvolley.VolleySingleton;

import java.util.HashMap;
import java.util.Map;

public class MultipartActivity extends Activity {

    private Context context = this;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Assuming you have prepared file data in fileData1 and fileData2
        String url = "http://192.168.1.100/api/postfile";
        MultipartRequest multipartRequest = new MultipartRequest(url, null, "multipart/form-data", multipartBody, new Response.Listener<NetworkResponse>() {
            @Override
            public void onResponse(NetworkResponse response) {
                Toast.makeText(context, "Upload successfully!", Toast.LENGTH_SHORT).show();
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Toast.makeText(context, "Upload failed!\r\n" + error.toString(), Toast.LENGTH_SHORT).show();
            }
        }) {
            @Override
            protected Map<String, String> getParams() {
                Map<String, String> params = new HashMap<>();
                params.put("text_field1", "Value for text field 1");
                params.put("text_field2", "Value for text field 2");
                return params;
            }

            @Override
            protected Map<String, DataPart> getByteData() {
                Map<String, DataPart> params = new HashMap<>();
                params.put("file_name1", new DataPart("file_name1.txt", "file content 1".getBytes(), "text/plain"));
                params.put("file_name2", new DataPart("file_name2.png", fileData1, "image/png"));
                return params;
            }
        };

        VolleySingleton.getInstance(context).addToRequestQueue(multipartRequest);
    }
}

import com.android.volley.AuthFailureError;
import com.android.volley.NetworkResponse;
import com.android.volley.ParseError;
import com.android.volley.Request;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.HttpHeaderParser;

import java.util.HashMap;
import java.util.Map;

class MultipartRequest extends Request<NetworkResponse> {

    private final Response.Listener<NetworkResponse> mListener;
    private final Response.ErrorListener mErrorListener;
    private Map<String, String> mHeaders;
    private Map<String, DataPart> mByteData;

    MultipartRequest(String url,
                     Map<String, String> headers,
                     String contentType,
                     Map<String, DataPart> byteData,
                     Response.Listener<NetworkResponse> listener,
                     Response.ErrorListener errorListener) {
        super(Method.POST, url, errorListener);
        this.mListener = listener;
        this.mErrorListener = errorListener;
        this.mHeaders = headers;
        this.mByteData = byteData;
    }

    @Override
    public Map<String, String> getHeaders() throws AuthFailureError {
        return mHeaders != null ? mHeaders : super.getHeaders();
    }

    @Override
    public String getBodyContentType() {
        return "multipart/form-data; boundary=" + getBoundary();
    }

    @Override
    public byte[] getBody() throws AuthFailureError {
        return encodeMultipartData(mByteData, getBoundary());
    }

    private String getBoundary() {
        return Long.toHexString(System.currentTimeMillis());
    }

    @Override
    protected Response<NetworkResponse> parseNetworkResponse(NetworkResponse response) {
        try {
            return Response.success(
                    response,
                    HttpHeaderParser.parseCacheHeaders(response));
        } catch (Exception e) {
            return Response.error(new ParseError(e));
        }
    }

    @Override
    protected void deliverResponse(NetworkResponse response) {
        mListener.onResponse(response);
    }

    @Override
    public void deliverError(VolleyError error) {
        mErrorListener.onErrorResponse(error);
    }

    protected static byte[] encodeMultipartData(Map<String, DataPart> dataParts,
                                                String boundary) {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        DataOutputStream dos = new DataOutputStream(bos);

        try {
            for (Map.Entry<String, DataPart> entry : dataParts.entrySet()) {
                dos.writeBytes(twoHyphens + boundary + lineEnd);
                dos.writeBytes("Content-Disposition: form-data; name=\"" +
                        entry.getKey() + "\"" +
                        "; filename=\"" +
                        entry.getValue().getFileName() + "\"" +
                        lineEnd);
                dos.writeBytes(String.format("Content-Type: %s%s", entry.getValue().getType(), lineEnd));
                dos.writeBytes(lineEnd);

                dos.write(entry.getValue().getContent());
                dos.writeBytes(lineEnd);
            }
            dos.writeBytes(lineEnd);
            dos.writeBytes(twoHyphens + boundary + twoHyphens + lineEnd);

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

    class DataPart {
        private String fileName;
        private byte[] content;
        private String type;

        DataPart(String fileName, byte[] content, String type) {
            this.fileName = fileName;
            this.content = content;
            this.type = type;
        }

        String getFileName() {
            return fileName;
        }

        byte[] getContent() {
            return content;
        }

        String getType() {
            return type;
        }
    }
}