判断上传的图片文件格式是否合法不是用后缀做的判断

/// <summary>
    /// 检查文件格式是否充许
    /// </summary>
    /// <param name="ImageFormat">待检查的文件格式</param>
    /// <returns>是否合法</returns>
    private bool HasThisForamt(string ImageFormat)
    {
        string ImageFormats = "gif|png|gif|bmp";
        if (ImageFormats.ToLower().IndexOf(ImageFormat.ToLower())>=0)
        {
            return true;
        }
        return false;
    }
    /// <summary>
    /// 判断上传的图片文件格式是否合法
    /// </summary>
    /// <param name="upControl">文件上传控件</param>
    /// <returns>是否合法</returns>
    private bool IsImageFormat( FileUpload upControl )
    {
        try
        {
            using (System.Drawing.Image img = System.Drawing.Image.FromStream(upControl.FileContent))
            {
                if (HasThisForamt("Bmp") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Bmp)) return true;
                if (HasThisForamt("Emf") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Emf)) return true;
                if (HasThisForamt("Exif") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Exif)) return true;
                if (HasThisForamt("Gif") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Gif)) return true;
                if (HasThisForamt("Icon") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Icon)) return true;
                if (HasThisForamt("Jpeg") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Jpeg)) return true;
                if (HasThisForamt("MemoryBmp") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.MemoryBmp)) return true;
                if (HasThisForamt("Png") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Png)) return true;
                if (HasThisForamt("Tiff") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Tiff)) return true;
                if (HasThisForamt("Wmf") && img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Wmf)) return true;
            }
        }
        catch
        {
            //此文件不是圖像文件
        }
        //您所選擇的文件格式不充许
        return false;
    }

//...........

    public bool IsEligibilityFileFormat(FileUpload fs)
    {
        bool xx = false;
        //System.IO.FileStream fs = new System.IO.FileStream(sFileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);
        System.IO.BinaryReader r = new System.IO.BinaryReader(fs.FileContent);
        string bx = " ";
        byte buffer;
        try
        {
            buffer = r.ReadByte();
            bx = buffer.ToString();
            buffer = r.ReadByte();
            bx += buffer.ToString();
        }
        catch (Exception exc)
        {
        }
        r.Close();
        //fs.Close();
        Response.Write(bx);
        if (bx == "7790 " || bx == "8297 " || bx == "8075 ")     //7790:exe,8297:rar,8075:pk  
        {
            xx = true;
        }
        return xx;
    /*
    7173 gif
    255216 jpg
    7790 exe dll
    00 ani--ico--cur
    7783
    255254 --ini
    9146 -- ini
    5866
    6395 hlp
    8269 reg
    70105 log
    205168
    7384 chm
    5549 txt
    117115 txt
    5450 txt
    5666 psd
    255254 rdp
    10056 bt种子
    8297 rar
    64101 bat
    */
    }