C# 根据文件流byte[]数组前两位 判断文件类型 附 文件扩展名说明

判断文件真实的类型，不是通过扩展名来判断：而是通过byte[]数组前两位编码来判断，
原文连接 https://www.cnblogs.com/Percy_Lee/p/4980451.html

         /// <summary>
        /// 判断文件格式
        /// http://www.cnblogs.com/babycool 
        /// </summary>
        /// <param name="filePath"></param>
        /// <returns></returns>
        public static bool IsAllowedExtension(string filePath)
        {
        FileStream stream </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> FileStream(filePath, FileMode.Open, FileAccess.Read);
        BinaryReader reader </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> BinaryReader(stream);
        </span><span style="color: #0000ff;">string</span> fileclass = <span style="color: #800000;">""</span><span style="color: #000000;">;
       </span><span style="color: #008000;">//</span><span style="color: #008000;"> byte buffer;</span>
        <span style="color: #0000ff;">try</span><span style="color: #000000;">
        {
            
            </span><span style="color: #008000;">//</span><span style="color: #008000;">buffer = reader.ReadByte();
            </span><span style="color: #008000;">//</span><span style="color: #008000;">fileclass = buffer.ToString();
            </span><span style="color: #008000;">//</span><span style="color: #008000;">buffer = reader.ReadByte();
            </span><span style="color: #008000;">//</span><span style="color: #008000;">fileclass += buffer.ToString();</span>

            <span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = <span style="color: #800080;">0</span>; i &lt; <span style="color: #800080;">2</span>; i++<span style="color: #000000;">)
            {
                fileclass </span>+=<span style="color: #000000;"> reader.ReadByte().ToString();
            }

        }
        </span><span style="color: #0000ff;">catch</span><span style="color: #000000;"> (Exception)
        {

            </span><span style="color: #0000ff;">throw</span><span style="color: #000000;">;
        }

        </span><span style="color: #0000ff;">if</span> (fileclass == <span style="color: #800000;">"</span><span style="color: #800000;">255216</span><span style="color: #800000;">"</span><span style="color: #000000;">)
        {
            </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">true</span><span style="color: #000000;">;
        }
        </span><span style="color: #0000ff;">else</span><span style="color: #000000;">
        {
            </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">false</span><span style="color: #000000;">;
        }

        </span><span style="color: #008000;">/*</span><span style="color: #008000;">文件扩展名说明
         * 255216 jpg
         * 208207 doc xls ppt wps
         * 8075 docx pptx xlsx zip
         * 5150 txt
         * 8297 rar
         * 7790 exe
         * 3780 pdf      
         * 
         * 4946/104116 txt
         * 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 
         * 4059        sgf    
         </span><span style="color: #008000;">*/</span><span style="color: #000000;">

    }</span></pre>