C# 跟进文件流分析文件后缀

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;

namespace WebApplication1.UserControl
{
    public partial class FileExtension : System.Web.UI.UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack) 
            {
                FileExt filetype = CheckExtension(@"C:\Users\**\Desktop\壁纸2");

                if (filetype == FileExt.PNG ||
                    filetype == FileExt.JPG ||
                    filetype == FileExt.GIF)
                {
                    Response.Write("这是一张图片,后缀为：" + filetype.ToString());
                }
                else
                {
                    Response.Write("非法文件");
                }
            }
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            if (!this.FileUpload1.HasFile) 
            {
                return;
            }
            System.IO.Stream fileStream;
            int FileLen;
            FileLen = this.FileUpload1.PostedFile.ContentLength;
            // 读取文件的 byte[] 
            byte[] bytes = new byte[FileLen];
            fileStream = this.FileUpload1.PostedFile.InputStream;
            fileStream.Read(bytes, 0, FileLen);
            fileStream.Seek(0, SeekOrigin.Begin);
            FileExt filetype = CheckExtension(fileStream);

            if (filetype == FileExt.PNG ||
                filetype == FileExt.JPG ||
                filetype == FileExt.GIF)
            {
                Response.Write("这是一张图片,后缀为：" + filetype.ToString());
            }
            else
            {
                Response.Write("非法文件");
            }

        }


        /// Checks the file is textfile or not.
        /// </summary>
        /// <param name="fileName">Name of the file.</param>
        /// <returns></returns>
        public static bool CheckIsTextFile(string fileName)
        {
            System.IO.FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
            bool isTextFile = true;
            try
            {
                int i = 0;
                int length = (int)fs.Length;
                byte data;
                while (i < length && isTextFile)
                {
                    data = (byte)fs.ReadByte();
                    isTextFile = (data != 0);
                    i++;
                }
                return isTextFile;
            }
            catch (Exception ex)
            {


                throw ex;
            }
            finally
            {
                if (fs != null)
                {
                    fs.Close();
                }
            }
        }

        public static FileExt CheckExtension(string fileName)
        {
            FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
            return CheckExtension(fs);
        }

        public static FileExt CheckExtension(Stream fs) 
        { 
            System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
            string fileType = string.Empty; 
            try
            {
                byte data = br.ReadByte();
                fileType += data.ToString();
                data = br.ReadByte();
                fileType += data.ToString();
                FileExt extension;
                try
                {
                    extension = (FileExt)Enum.Parse(typeof(FileExt), fileType);
                }
                catch
                {

                    extension = FileExt.VALIDFILE;
                }
                return extension;
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                if (fs != null)
                {
                    fs.Close();
                    br.Close();
                }
            }
        }
    }

   
}

public enum FileExt
{
    JPG = 255216,
    GIF = 7173,
    PNG = 13780,
    SWF = 6787,
    RAR = 8297,
    ZIP = 8075,
    _7Z = 55122,
    VALIDFILE = 9999999
    // 255216 jpg; 

    // 7173 gif; 

    // 6677 bmp, 

    // 13780 png; 

    // 6787 swf 

    // 7790 exe dll, 

    // 8297 rar 

    // 8075 zip 

    // 55122 7z 

    // 6063 xml 

    // 6033 html 

    // 239187 aspx 

    // 117115 cs 

    // 119105 js 

    // 102100 txt 

    // 255254 sql  

}