当前上下文中不存在“MimeTypeMap”

当前上下文中不存在“MimeTypeMap” 怎么引用进来 应该下载什么扩展呢

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using AisinIX.Amateras.Common.Models;
using AisinIX.Amateras.Common.Utilities;
using AisinIX.Amateras.UserInfo.Common.Models;
using AisinIX.CMS.CommonMaster.Models;
using AisinIX.CMS.EnvironmentMaster.services;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Http.Extensions;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using AisinIX.Amateras.UserInfo.Common.ServiceAccessors;

namespace AisinIX.CMS.EnvironmentMaster.WebApi.Controllers
{
    [ApiController]
    [Route("api/environmentMaster")]
    public class EnvironmentMasterController : ControllerBase
    {
        private readonly ILogger _logger;
        private readonly IConfigUtility _configUtility;
        private readonly IApiContext _apiContext;
        private IEnvironmentMasterService _EnvironmentMasterService;
        private readonly IUserInfoServiceAccessor _userInfoServiceAccessor;

        public EnvironmentMasterController(
            ILogger logger,
            IApiContext apiContext, 
            IConfigUtility configUtility,
            IEnvironmentMasterService EnvironmentMasterService,
            IUserInfoServiceAccessor userInfoServiceAccessor
        )
        {
            _logger = logger;
            _configUtility = configUtility;
            _apiContext = apiContext;
            _EnvironmentMasterService = EnvironmentMasterService;
            _userInfoServiceAccessor = userInfoServiceAccessor;
        }
        public ActionResult GetMasterDownLoad([FromQuery(Name = "masterName")] string masterName){
            try
            {
                String URL = HttpContext.Request.GetDisplayUrl();

                _logger.LogInformation("┏■ GetMasterDownLoad 開始 ■");
                _logger.LogInformation($"URL:{URL}");
                _logger.LogInformation($"各種マスタデータのダウンロード(マスタ=[{masterName}])");  

                if (String.IsNullOrEmpty(masterName))
                {
                    _logger.LogWarning($"マスタ名がブランクです。");
                    return BadRequest();
                } 
                masterName = masterName.Trim(); 

                MemoryStream result = _EnvironmentMasterService.MasterDataDownLoad(masterName,"0");

                if(result == null){
                    return StatusCode(404);
                }

                string fileName = masterName + ".xlsx";
                // 说这个在上下文中不存在 “MimeTypeMap” 
                string contentType = MimeTypeMap.GetMimeType(fileName);
                
                var bytes = result.ToArray();
                var cd = new System.Net.Mime.ContentDisposition
                {
                    FileName = fileName,
                    Inline = false
                };

                Response.Headers.Add("Content-Disposition", cd.ToString());

                return File(bytes, contentType);

            }
            catch (Exception ex)
            {
                int eventlogID = int.Parse(_configUtility.GetStringSetting("EventLogID"));
                _logger.LogError(eventlogID, $"エラーが発生({ex.Message})");
                return StatusCode(500);
            }
            finally
            {
                _logger.LogInformation("┗■ GetMasterDownLoad 終了 ■");
            }
        }
    }
}