怎样输出图片下载?

ASP.NET可以直接输出文件下载,例如doc,zip,rar这类IIS不能解释的格式,如果是Bmp,Gif,Jepg就不行了,怎样实现其下载呢(出现保存那个对话框)?

// Identify the file to download including its path.

string filepath = DownloadFileName;

// Identify the file name.

string filename = System.IO.Path.GetFileName(filepath);

Response.Clear();

// Specify the Type of the downloadable file.

Response.ContentType = "application/octet-stream";

// Set the Default file name in the FileDownload dialog box.

Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);

Response.Flush();

// Download the file.

Response.WriteFile(filepath);