前端程式碼

 /**
     * @param {Object} data
     * @param {Object} fileName
     * @description 轉換格式後下載
     */
    download(data, fileName) {
      if (!data) return;
      console.log(data);
      const url = window.URL.createObjectURL(new Blob([data]));
      let link = document.createElement('a');
      link.style.display 
 
= 'none';
      link.href = url;
      link.setAttribute('download', fileName);
      document.body.appendChild(link);
      link.click();
    },
    /**
     * @description 下載檔案
     */
    async downFile(row) {
      const res = await this.$ajax({
        method: 'get',
        url: `/5002/File/down/${row.id}`,
        responseType: 
 
'blob'
      })
      this.download(res, row.name)
    },

後端程式碼

 private FileService _fileService;
        /// <summary>
        /// 檔案服務
        /// </summary>
        /// <param name="fileService"></param>
        public FileController(FileService fileService)
        {
            _fileService 
 
= fileService;
        }

        private readonly List<string> Images = new List<string>() { "jpg", "png", "bmp", "jpge" };
        private readonly List<string> Files = new List<string>() { "txt", "doc", "docx", "xls", "pdf", "xlsx" };

        /// <summary>
        /// 上傳圖片（JPG,PNG,BMP等圖片檔案）
        /// </summary>
        /// <returns></returns>
        [HttpPost("upload-img")]
        public async Task<BaseResponse<List<SysFile>>> UploadImg()
        {
            var files = Request.Form.Files;
            if (files.Count == 0)
            {
                throw new Exception("沒有上傳檔案");
            }
            foreach(var file in files)
            {
                var fileName = Path.GetFileName(file.FileName);
                //取得檔案的副檔名
                var  fileExtension = Path.GetExtension(fileName).ToLower();
                if (string.IsNullOrEmpty(fileExtension))
                {
                    throw new Exception("對不起，只能上傳圖片檔案");
                }
                fileExtension = fileExtension.Replace(".", "");
                if (!Images.Contains(fileExtension))
                {
                    throw new Exception($"對不起，只能上傳圖片格式檔案，檔案型別為 [ {string.Join(",", Images)} ] 中的一種");
                }
            }
            var result = await this.Upload("img", files);
            return BaseResponse.GetResult(result);
        }

        /// <summary>
        /// 上傳檔案（PDF,WORD,EXCEL,TXT等非圖片類檔案）
        /// </summary>
        /// <returns></returns>
        [HttpPost("upload-file")]
        public async Task< BaseResponse<List<SysFile>>> UploadFile()
        {

            var files = Request.Form.Files;
            if (files.Count == 0)
            {
                throw new Exception("沒有上傳檔案");
            }
            foreach (var file in files)
            {
                var fileName = Path.GetFileName(file.FileName);
                //取得檔案的副檔名
                var fileExtension = Path.GetExtension(fileName).ToLower();
                if (string.IsNullOrEmpty(fileExtension))
                {
                    throw new Exception("對不起，只能上傳文件類檔案");
                }
                fileExtension = fileExtension.Replace(".", "");
                if (!Files.Contains(fileExtension))
                {
                    throw new Exception($"對不起，只能上傳文件格式檔案，檔案型別為 [ {string.Join(",", Files)} ] 中的一種");
                }
            }
            var result = await this.Upload("file", files);
            return  BaseResponse.GetResult(result);
        }

        /// <summary>
        /// 上傳 並將記錄儲存到資料庫中
        /// </summary>
        /// <param name="type"></param>
        /// <param name="files"></param>
        /// <returns></returns>
        private async Task<List<SysFile>> Upload(string type,IFormFileCollection files)
        {
            List<SysFile> result = new List<SysFile>();

            string dir = DateTime.Now.ToString("yyyyMMdd");
            string savePath = Path.Combine(AppContext.BaseDirectory, "wwwroot", "upload", type, dir);
            if (!Directory.Exists(savePath))
            {
                Directory.CreateDirectory(savePath);
            }

            foreach (var file in files)
            {
                if (file.Length > 0)
                {
                    string fileName, fileExtension;
                    //取得上傳得檔名
                    fileName = Path.GetFileName(file.FileName);
                    //取得檔案的副檔名
                    fileExtension = Path.GetExtension(fileName).ToLower();
                    
                    string newFileName = $"{ Guid.NewGuid().ToString()}{ fileExtension}";
                    using (var stream = new FileStream(Path.Combine(savePath, newFileName), FileMode.Create))
                    {
                        file.CopyToAsync(stream).Wait();
                        var filePath = $"/upload/{type}/{dir}/{newFileName}";

                        result.Add(new SysFile()
                        {
                            FileEx = fileExtension,
                            FileName = fileName,
                            Path = filePath,
                            UploadUser = User.Identity.Name
                        });
                    }
                }
            }
            result = await  _fileService.CreateAsync(result);

            return result;
        }

        /// <summary>
        /// 獲取檔案詳情
        /// </summary>
        /// <param name="id">SysFile.ID</param>
        /// <returns></returns>
        [HttpGet("file-info/{id}")]
        public BaseResponse<SysFile> GetFileInfo(string id)
        {
            var file = _fileService.Get(id);
            return BaseResponse.GetResult(file);
        }

        /// <summary>
        /// 檔案下載
        /// </summary>
        /// <param name="id">SysFile.ID</param>
        /// <returns>application/octet-stream</returns>
        [HttpGet("down/{id}")]
        public IActionResult Down(string id)
        {
            var file = _fileService.Get(id);
            var paths = file.Path.Split("/").ToList();
            paths.Remove(paths.First());
            paths.Insert(0, AppContext.BaseDirectory);
            paths.Insert(1, "wwwroot");
            var filePath = Path.Combine(paths.ToArray());
            if (System.IO.File.Exists(filePath))
            {
                return PhysicalFile(filePath, "application/octet-stream", file.FileName);
            }
            else
            {
                throw new Exception($"下載失敗！未能找到指定檔案，檔案ID:{id},Path:{file.Path}");
            }
        }
    }