You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: goods_api.go
    path of file: ./repos/gopay/alipay
    
    the code of the file until where you have to start completion: package alipay

import (
	"context"
	"encoding/json"
	"fmt"

	"github.com/go-pay/gopay"
)

// alipay.merchant.item.file.upload(商品文件上传接口)
// 文档地址：https://opendocs.alipay.com/apis/api_4/alipay.mer
    
func (a *Client) MerchantItemFileUpload(ctx context.Context, file *gopay.File) (aliRsp *MerchantItemFileUploadRsp, err error) {
	bm := make(gopay.BodyMap)
	bm.Set("scene", "SYNC_ORDER") //素材固定值

	var bs []byte
	if bs, err = a.FileRequest(ctx, bm, file, "alipay.merchant.item.file.upload"); err != nil {
		return nil, err
	}
	aliRsp = new(MerchantItemFileUploadRsp)
	if err = json.Unmarshal(bs, aliRsp); err != nil {
		return nil, err
	}
	if aliRsp.Response != nil && aliRsp.Response.Code != "10000" {
		info := aliRsp.Response
		return aliRsp, fmt.Errorf(`{"code":"%s","msg":"%s","sub_code":"%s","sub_msg":"%s"}`, info.Code, info.Msg, info.SubCode, info.SubMsg)
	}
	signData, signDataErr := a.getSignData(bs, aliRsp.AlipayCertSn)
	aliRsp.SignData = signData
	return aliRsp, a.autoVerifySignByCert(aliRsp.Sign, signData, signDataErr)
}