api

@Multipart
    @PUT("/users/me")
    Observable<UserResponse> updateUser(
            @PartMap Map<String, Object> userData,
            @Part("user[avatar]") TypedFile avatar
    );

call

mApi.updateUser(request.getUser().getDataAsMap(null, null), request.getAvatar())
                .compose(RxUtils.applySchedulersAndRetry())

gistfile1.txt

package com.bookjane.models;

import android.support.annotation.NonNull;
import android.support.annotation.Nullable;

import com.google.gson.annotations.SerializedName;

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;

/**
 * Created by rkom on 19.04.16.
 */
public abstract class BasePartMap implements Reflectable {

    private final static String MAP_POSTFIX = "]";

    private final String mMapPrefix = getFieldMapPrefix();

    @NonNull
    public abstract String getFieldMapPrefix();

    @Override
    public Map<String, Object> getDataAsMap(@Nullable String parentPrefix,
                                            @Nullable String parentPostfix) {
        final String prefix = parentPrefix == null ? mMapPrefix : parentPrefix + mMapPrefix;
        final String postfix = parentPostfix == null ? MAP_POSTFIX : parentPostfix + MAP_POSTFIX;
        Field[] fields = this.getClass().getDeclaredFields();
        Map<String, Object> map = new HashMap<>();
        Object value;
        SerializedName serialName;
        for (Field field : fields) {
            value = getFieldValue(field);
            if (value != null) {
                if (value instanceof Reflectable) {
                    map.putAll(((Reflectable) value).getDataAsMap(prefix, postfix));
                    continue;
                }
                serialName = field.getAnnotation(SerializedName.class);
                if (serialName != null) {
                    map.put(prefix + serialName.value() + postfix, getFieldValue(field));
                }
            }
        }
        return map;
    }

    @Nullable
    private Object getFieldValue(@NonNull Field field) {
        try {
            field.setAccessible(true);
            if (field.getType().isEnum()) {
                return field.get(this) == null ? null : field.get(this).toString();
            } else {
                return field.get(this);
            }
        } catch (IllegalAccessException e) {
            return null;
        }
    }
}

instance

public class User extends BasePartMap implements Parcelable {

    private final static String MAP_PREFIX = "user[";
    private final static int INT_NULL_FLAG = -1; // for Integer vars

    @NonNull
    @Override
    public String getFieldMapPrefix() {
        return MAP_PREFIX;
    }
}