mime中间件Demo，里面用到的有

1.path模块 //引入模块

var path=require('path');

2.extname方法 //获取文件的扩展名

var extname=path.extname(realPath)

var path=require('path');    mime={        "html": "text/html",        "js": "text/javascript",        "css": "text/css",        "xml": "text/xml",        "txt": "text/plain",        "ico": "image/x-icon",        "svg": "image/svg+xml",        "gif": "image/gif",        "png": "image/png",        "jpg": "image/jpeg",        "jpeg": "image/jpeg",        "tiff": "image/tiff",        "json": "application/json",        "pdf": "application/pdf",        "swf": "application/x-shockwave-flash",        "wav": "audio/x-wav",        "wma": "audio/x-ms-wma",        "wmv": "video/x-ms-wmv"    };module.exports=function (realPath){    var extname=path.extname(realPath),        ext=extname?extname.slice(1):'unknown',        contentType=mime[ext] ||'text/plain';    return contentType;};