自己动手写了这么一个demo。并且支持客户端判断文件大小和文件格式。其实原理就是通过各种版本的浏览器实行不同的实现机制。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Firefox3,IE6,IE7,IE8上传图片预览</title><style type="text/css">#preview_wrapper{ display:inline-block; width:300px; height:300px; background-color:#CCC;}#preview_fake{ /* 该对象用于在IE下显示预览图片 */ filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale);}#preview_size_fake{ /* 该对象只用来在IE下获得图片的原始尺寸，无其它用途 */ filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=image); visibility:hidden;}#preview{ /* 该对象用于在FF下显示预览图片 */ width:300px; height:300px;}</style><script type="text/javascript">function onUploadImgChange(sender){ var filePath = sender.value; var fileExt = filePath.substring(filePath.lastIndexOf(".")).toLowerCase(); if(!checkFileExt(fileExt)) { alert("您上传的文件不是图片,请重新上传！"); return; } var objPreview = document.getElementById( 'preview' ); var objPreviewFake = document.getElementById( 'preview_fake' ); var objPreviewSizeFake = document.getElementById( 'preview_size_fake' ); try{ if( sender.files && sender.files[0] ){ objPreview.style.display = 'block'; objPreview.style.width = 'auto'; objPreview.style.height = 'auto'; // Firefox 因安全性问题已无法直接通过 input[file].value 获取完整的文件路径 objPreview.src = sender.files[0].getAsDataURL(); //读取图片文件大小 fileSize = sender.files[0].fileSize; checkFileSize(sender,fileSize); }else if( objPreviewFake.filters ){ // IE7,IE8 在设置本地图片地址为 img.src 时出现莫名其妙的后果 //（相同环境有时能显示，有时不显示），因此只能用滤镜来解决 // IE7, IE8因安全性问题已无法直接通过 input[file].value 获取完整的文件路径 sender.select(); var imgSrc = document.selection.createRange().text; objPreviewFake.filters.item( 'DXImageTransform.Microsoft.AlphaImageLoader').src = imgSrc; objPreviewSizeFake.filters.item( 'DXImageTransform.Microsoft.AlphaImageLoader').src = imgSrc; autoSizePreview( objPreviewFake, objPreviewSizeFake.offsetWidth, objPreviewSizeFake.offsetHeight ); objPreview.style.display = 'none'; //读取图片文件大小 var sh = setInterval( function(){ var img = document.createElement("img"); img.src = imgSrc; fileSize = img.fileSize; if (fileSize > 0){ checkFileSize(sender,fileSize);clearInterval(sh);} img = null; } ,100); } }catch(e) { alert("您上传的文件过大或者不是图片,请重新上传！"); } }function onPreviewLoad(sender){ autoSizePreview( sender, sender.offsetWidth, sender.offsetHeight );}function autoSizePreview( objPre, originalWidth, originalHeight ){ var zoomParam = clacImgZoomParam( 300, 300, originalWidth, originalHeight ); objPre.style.width = zoomParam.width + 'px'; objPre.style.height = zoomParam.height + 'px'; objPre.style.marginTop = zoomParam.top + 'px'; objPre.style.marginLeft = zoomParam.left + 'px';}function clacImgZoomParam( maxWidth, maxHeight, width, height ){ var param = { width:width, height:height, top:0, left:0 }; if( width>maxWidth || height>maxHeight ){ rateWidth = width / maxWidth; rateHeight = height / maxHeight; if( rateWidth > rateHeight ){ param.width = maxWidth; param.height = height / rateWidth; }else{ param.width = width / rateHeight; param.height = maxHeight; } } param.left = (maxWidth - param.width) / 2; param.top = (maxHeight - param.height) / 2; return param;}function checkFileSize(sender,fileSize){ if(fileSize > 3145728) { alert("您上传的文件过大,请上传小于3M的图片！"); return false; } return true;}function checkFileExt(ext){ if (!ext.match(/.jpg|.gif|.png|.bmp/i)) { return false; } return true;}</script></head><body> <div id="preview_wrapper"> <div id="preview_fake"> <img id="preview" onload="onPreviewLoad(this)"/> </div> </div> <br/> <input id="upload_img" type="file" onchange="onUploadImgChange(this)"/> <br/> <img id="preview_size_fake"/></body></html>
来源：https://www.cnblogs.com/doublemm/archive/2011/08/09/2132715.html