本文摘自PHP中文网，作者黄舟，侵删。

本文主要用到的知识

　　HTML5中的File及FileReader接口

源码

<!DOCTYPE html><html><head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title>HTML5文件拖拽预览Demo</title>
    <style type="text/css">
        h1{
            padding:0px;
            margin:0px;
        }
        p#show{
            border: 1px solid #ccc; 
            width: 400px; 
            height: 300px;
            display: -moz-box;
            display: -webkit-box;
            -moz-box-align: center;
            -webkit-box-align: center;
            -moz-box-pack: center;
            -webkit-box-pack: center;
            resize:both;
            overflow:auto;
        }
        p[id^=show]:hover{
            border: 1px solid #333; 
        }
        p#main{
            width:100%;
        }
        p#successLabel        {
            color:Red;
        }
        p#content        {
            display:none;
        }
    </style>
    <script type="text/javascript">
        function init() 
        {            var dest = document.getElementById("show");
            dest.addEventListener("dragover", function(ev) 
            {
                ev.stopPropagation();
                ev.preventDefault();
            }, false);
         
            dest.addEventListener("dragend", function(ev) 
            {
                ev.stopPropagation();
                ev.preventDefault();
            }, false);
         
            dest.addEventListener("drop", function (ev) {
                ev.stopPropagation();
                ev.preventDefault();        
                var file = ev.dataTransfer.files[0];                
                var reader = new FileReader();        
                if (file.type.substr(0, 5) == "image") {
                    reader.onload = function (event) {
                        dest.style.background = 'url(' + event.target.result + ') no-repeat center';
                        dest.innerHTML = "";
                    };
                    reader.readAsDataURL(file);
                }                else if (file.type.substr(0, 4) == "text") {
         
                    reader.readAsText(file);
                    reader.onload = function (f) {
                        dest.innerHTML = "<pre>" + this.result + "</pre>";
                        dest.style.background = "white";
                    }
                }                else {
                    dest.innerHTML = "暂不支持此类文件的预览";
                    dest.style.background = "white";
                }
            }, false);
        }        
        //设置页面属性，不执行默认处理（拒绝被拖放）        
        document.ondragover = function(e){e.preventDefault();};
        document.ondrop = function(e){e.preventDefault();}
         
        window.onload=init;    
        </script></head><body>
    <h1>HTML5文件拖拽预览Demo</h1>
    <p id="show">
    文件预览区，仅限图片和txt文件    
    </p></body></html>