Java Nested Multipart

EntityBody.java

package com.jooyunghan;

import org.apache.http.HttpEntity;
import org.apache.http.entity.mime.content.ContentBody;

import java.io.IOException;
import java.io.OutputStream;

public class EntityBody implements ContentBody {
    private final HttpEntity entity;

    public EntityBody(HttpEntity entity) {
        this.entity = entity;
    }

    @Override
    public String getFilename() {
        return null;
    }

    @Override
    public void writeTo(OutputStream out) throws IOException {
        entity.writeTo(out);
    }

    @Override
    public String getMimeType() {
        return entity.getContentType().toString();
    }

    @Override
    public String getMediaType() {
        return null;
    }

    @Override
    public String getSubType() {
        return null;
    }

    @Override
    public String getCharset() {
        return null;
    }

    @Override
    public String getTransferEncoding() {
        return "binary";
    }

    @Override
    public long getContentLength() {
        return entity.getContentLength();
    }
}

Main.java

package com.jooyunghan;

import com.google.gson.Gson;
import org.apache.http.HttpEntity;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.mime.MultipartEntityBuilder;

import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

public class Main {

    public static void main(String[] args) throws IOException {
        Gson gson = new Gson();

        Map<String, Object> meta = new TreeMap<>();
        meta.put("key", "value");
        meta.put("키", "값");
        meta.put("서브", Map.of("키", List.of(1, true)));

        final String filename = "한글.png";

        Charset utf8 = Charset.forName("utf-8");
        HttpEntity body = MultipartEntityBuilder.create()
                .setLaxMode()
                .setCharset(utf8)
                .addTextBody("filename", filename, ContentType.TEXT_PLAIN.withCharset(utf8))
                .addBinaryBody("file", new File(filename))
                .build();

        HttpEntity entity = MultipartEntityBuilder.create()
                .addTextBody("header", gson.toJson(meta), ContentType.APPLICATION_JSON)
                .addPart("body", new EntityBody(body))
                .build();

        System.out.println(entity.getContentType() + "\n");
        entity.writeTo(System.out);
    }

}