Three ways to determine the type of uploaded files in asp.net

　1. The security is relatively low. Changing the text file 1.txt to 1.jpg can still be uploaded, but the implementation method is easy to understand and simple to implement, so many people on the Internet still use this method.

Boolean fileOk = false;
          string path = Server.MapPath("~/images/");
          //判断是否已经选取文件
          if (FileUpload1.HasFile)
          {
              //取得文件的扩展名,并转换成小写
              string fileExtension = System.IO.Path.GetExtension(FileUpload1.FileName).ToLower();
              //限定只能上传jpg和gif图片
              string[] allowExtension = { ".jpg", ".gif" };
              //对上传的文件的类型进行一个个匹对
              int j = 0;
              for (int i = 0; i < allowExtension.Length; i++)
              {
                  if (fileExtension == allowExtension[i])
                  {
                      fileOk = true;
                      return;
                  }
                  else
                  {
                      j++;
                  }
              }
              if (j > 0)
              {
                  Response.Write("<script>alert(&#39;文件格式不正确&#39;);</script>");
                  return;
              }
          }
          else
          {
              Response.Write("<script>alert(&#39;你还没有选择文件&#39;);</script>");
              return;
          }
          //如果扩展名符合条件，则上传
          if (fileOk)
          {
              FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);
              Response.Write("<script>alert(&#39;上传成功&#39;);</script>");
          }

Second, do not detect the file suffix but detect the file MIME content type.

Boolean fileOk = false;
           string path = Server.MapPath("~/images/");
           //判断是否已经选取文件
           if (FileUpload1.HasFile)
           {
               //取得文件MIME内容类型
               string type = this.FileUpload1.PostedFile.ContentType.ToLower();
               if (type.Contains("image"))    //图片的MIME类型为"image/xxx"，这里只判断是否图片。
               {
                   fileOk = true;
 
               }
               else
               {
                   Response.Write("<script>alert(&#39;格式不正确&#39;)</script>");
               }
           }
           else
           {
               Response.Write("<script>alert(&#39;你还没有选择文件&#39;);</script>");
           }
           //如果扩展名符合条件，则上传
           if (fileOk)
           {
               FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);
               Response.Write("<script>alert(&#39;上传成功&#39;);</script>");
           }

3. Can realize file type judgment in the true sense

try
            {
                //判断是否已经选取文件
                if (FileUpload1.HasFile)
                {
                    if (IsAllowedExtension(FileUpload1))
                    {
                        string path = Server.MapPath("~/images/");
                        FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);
                        Response.Write("<script>alert(&#39;上传成功&#39;);</script>");
                    }
                    else
                    {
                        Response.Write("<script>alert(&#39;您只能上传jpg或者gif图片&#39;);</script>");
                    }
 
                }
                else
                {
                    Response.Write("<script>alert(&#39;你还没有选择文件&#39;);</script>");
                }
            }
            catch (Exception error)
            {
                Response.Write(error.ToString());
            }
            #endregion
        }
//真正判断文件类型的关键函数
        public static bool IsAllowedExtension(FileUpload hifile)
        {
            System.IO.FileStream fs = new System.IO.FileStream(hifile.PostedFile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);
            System.IO.BinaryReader r = new System.IO.BinaryReader(fs);
            string fileclass = "";
            //这里的位长要具体判断.
            byte buffer;
            try
            {
                buffer = r.ReadByte();
                fileclass = buffer.ToString();
                buffer = r.ReadByte();
                fileclass += buffer.ToString();
 
            }
            catch
            {
 
            }
            r.Close();
            fs.Close();
            if (fileclass == "255216" || fileclass == "7173")//说明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar
            {
                return true;
            }
            else
            {
                return false;
            }
 
        }