检测真实的文件类型函数

private bool IsAllowedExtension(HttpPostedFile hifile)
        {
            bool ret = false;

            System.IO.FileStream fs = new System.IO.FileStream(hifile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);
            System.IO.BinaryReader r = new System.IO.BinaryReader(fs);
            string fileclass = "";
            byte buffer;
            try
            {
                buffer = r.ReadByte();
                fileclass = buffer.ToString();
                buffer = r.ReadByte();
                fileclass += buffer.ToString();
            }
            catch
            {
                return false;
            }
            r.Close();
            fs.Close();
            /*文件扩展名说明
             *7173        gif 
             *255216      jpg
             *13780       png
             *6677        bmp
             *239187      txt,aspx,asp,sql
             *208207      xls.doc.ppt
             *6063        xml
             *6033        htm,html
             *4742        js
             *8075        xlsx,zip,pptx,mmap,zip
             *8297        rar   
             *01          accdb,mdb
             *7790        exe,dll           
             *5666        psd 
             *255254      rdp 
             *10056       bt种子 
             *64101       bat 
             */


            String[] fileType = { "255216", "7173", "6677", "13780", "8297", "5549", "870", "87111", "8075" };

            for (int i = 0; i < fileType.Length; i++)
            {
                if (fileclass == fileType[i])
                {
                    ret = true;
                    break;
                }
            }
            return ret;       
        }