C#检测上传文件类型（真实检测，不通过后缀名）

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//

        /// C#检测真实文件类型函数
        ///
        ///
        ///
        public static 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(hifile.InputStream);
            //hifile.InputStream
            string fileclass =  "" ;
            byte buffer;
            try
            {
                buffer = r.ReadByte();
                fileclass = buffer.ToString();
                buffer = r.ReadByte();
                fileclass += buffer.ToString();
            }
            catch
            {
                return false ;
            }
            r.Close();