主要是用来判断客户端上传的图片是否为真实的图片，以防病毒侵入，保证上传的文件的安全。

主要代码如下：

需要引用

using System.IO;

public void UploadFile()
{
try
            { 
                HttpPostedFile postfile = Request.Files["file"]; 
                string savepath = Server.MapPath("Image/" + postfile.FileName);
                postfile.SaveAs(savepath); 
                FileStream fs = new FileStream(savepath, FileMode.Open, FileAccess.Read);
                BinaryReader reader = new BinaryReader(fs); 
                string fileClass; 
                byte buffer; 
                byte[] b = new byte[2]; 
                buffer = reader.ReadByte(); 
                b[0] = buffer; 
                fileClass = buffer.ToString(); 
                buffer = reader.ReadByte(); 
                b[1] = buffer; 
                fileClass += buffer.ToString();  
                reader.Close(); 
                fs.Close();
                
                if (fileClass == "255216" || fileClass == "7173" || fileClass == "6677" || fileClass == "13780")
                {
                    //255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar 
                    //Response.Write("图片可用"); 
                    //保存到数据库中
                } 
                else
                { 
                    //Response.Write("图片非法"); 
                    File.Delete(savepath); //删除文件
                    return; 
                } 
            } 
            catch (Exception)
            { //Response.Write("图片非法！"); 
                return; 
                throw; 
            }
}

/// <summary>
        /// 上传头像
        /// </summary>
        /// <param name="userId">用户编号</param>
        /// <returns>Json(-1表示系统异常，-2表示文件不合法)</returns>
        [HttpPost] 
        public JsonResult UploadAvatar(string userId)
        {
            //上传头像
            string folderPath = "/upload/avatar/";
            //判断路径是否存在
            if (!Directory.Exists(folderPath))
                Directory.CreateDirectory(folderPath);//创建文件路径
            HttpPostedFileBase uploadFile = Request.Files["avatars"];
            if (uploadFile != null)
            {
                string oriFileName = uploadFile.FileName;//原始文件名
                string fileName = userId + "_" + oriFileName;
                uploadFile.SaveAs(Server.MapPath(folderPath + fileName));
                FileStream fs = new FileStream(Server.MapPath(folderPath + fileName), FileMode.Open, FileAccess.Read);
                BinaryReader reader = new BinaryReader(fs);
                string fileClass;
                byte buffer;
                byte[] b = new byte[2];
                buffer = reader.ReadByte();
                b[0] = buffer;
                fileClass = buffer.ToString();
                buffer = reader.ReadByte();
                b[1] = buffer;
                fileClass += buffer.ToString();
                reader.Close();
                fs.Close();
                if (fileClass == "255216" || fileClass == "7173" || fileClass == "6677" || fileClass == "13780")
                {
                    //255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar 
                    //Response.Write("图片可用"); 
                    //保存到数据库中
                }
                else
                {
                   
                    //Response.Write("图片非法"); 
                    File