function validateFileSize(id, maxsize) {
            var btnsave = document.getElementById("Button2");
            btnsave.disabled = false;
            var filepath = "";
            var fileupload = document.getElementById(id);
            if (fileupload.value.length < 5) { alert('请选择文件！'); return; }
            var agent = window.navigator.userAgent;
            if (document.all) {
                var isIE7 = agent.indexOf('MSIE 7.0') != -1;
                var isIE8 = agent.indexOf('MSIE 8.0') != -1;
                //IE7和IE8获得文件路径
                if (isIE7 || isIE8) {
                    fileupload.select();
                    filepath = document.selection.createRange().text;
                }
                //IE6获得文件路径
                else
                { filepath = file.value; }
                //PageMethods.ValidateFile(filepath, maxsize, ieCallBack);
                alert(filepath);
            }
            if (agent.indexOf("Firefox") >= 1) {
                alert("FF");
                alert(getValue());
                return false;
                if (fileupload.files) {
                    var size = fileupload.files[0].fileSize;
                    if (size > parseInt(maxsize)) {
                        fileupload.parentNode.innerHTML = '<input type="file" name="' + fileupload.name + '" id="' + fileupload.id + '" onchange="disableSave(this);"/> ';
                        alert("文件超过大小！");
                    }
                    else { alert("文件上传成功！"); }
                }
            }
        }

/**
*函数描述：获取input type=file的图像全路径
* @obj input type=file的对象
**/
function getFullPath(obj)
{
if(obj)
{
//ie
if (window.navigator.userAgent.indexOf("MSIE")>=1)
{
obj.select();
return document.selection.createRange().text;
}
//firefox
else if(window.navigator.userAgent.indexOf("Firefox")>=1)
{
if(obj.files)
{
return obj.files.item(0).getAsDataURL();
}
return obj.value;
}
return obj.value;
}
}

<input type="file" onchange="document.getElementById('img').src=getFullPath(this);" >

#判断IE还是Firefox

function getFullPath(obj)

{

if(obj)

{

//ie

if (window.navigator.userAgent.indexOf("MSIE")>=1)

{

obj.select();

return document.selection.createRange().text;

}

//firefox

else if(window.navigator.userAgent.indexOf("Firefox")>=1)

{

if(obj.files)

{

return obj.files.item(0).getAsDataURL();

}

return obj.value;

}

return obj.value;

}

}

#判断后缀名

function yulan(){

var filePath =getFullPath(document.getElementById('UpFile'));

var fileText =filePath.substring(filePath.lastIndexOf("."),filePath.length);

var fileName =fileText.toLowerCase();

if ((fileName!='.jpg')&&(fileName!='.gif')&&(fileName!='.jpeg')&&(fileName!='.png')&&(fileName!='.bmp'))

{

alert("对不起，系统仅支持标准格式的照片，请您调整格式后重新上传，谢谢 ！");

document.form1.UpFile.focus();

}

else

{

document.getElementById("preview").innerHTML="<img src='"+getFullPath(document.getElementById('UpFile'))+"' width=120 style='border:6px double #ccc'>";

}

}

#判断文件大小

var Sys = {};

if(navigator.userAgent.indexOf("MSIE")>0) {

Sys.ie=true;

}

if(isFirefox=navigator.userAgent.indexOf("Firefox")>0){

Sys.firefox=true;

}

var maxLength = 5*1024*1024;

function checkFileChange(obj) {

var filesize = 0;

if(Sys.firefox){

filesize = obj.files[0].fileSize;

}else if(Sys.ie){

var filePath = obj.value;

var image=new Image();

image.src=filePath;

filesize=image.fileSize;

}

alert(filesize);

}

<input id="file_upl" type="file" />

var file_upl = document.getElementById('file_upl');
file_upl.select();

var realpath = document.selection.createRange().text;

var file_upl = document.getElementById('file_upl-file');
file_upl.select();

var realpath = document.selection.createRange().text;