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js判断图片大小与图片类型<script language="javascript">
function checkinput(){
var sUploadAllowedExtensions  = ".gif .jpg .jpeg .png" ;
var sExt = document.file_upload.file.value.match( /.[^.]*$/ ) ;
sExt = sExt ? sExt[0].toLowerCase() : "." ;

if (document.file_upload.file.value==""){
   alert("文件路径不能为空！");
   return false;
   }
 
 if ( sUploadAllowedExtensions.indexOf( sExt ) < 0 )
 {
  alert("对不起, 只有下面的图片格式才能上传:nn" + sUploadAllowedExtensions + "nn请重新选择.") ;
  return false;
 }
 var ipreview = document.all("ipreview")
 if (ipreview.fileSize>10240){
  alert("您选择的图片尺寸大于"+Math.round(10240/1024)+"KB，不能上传");
  return false;
  //myddd.disabled=true;
  }
  return true;
}

function isImage(file){
 //Get a file extension
 var ext = file.substr(file.lastIndexOf(".")).toLowerCase()

 //Check extension to image types.
 
 return ".gif,.jpg,.png,.jpeg,.ico,".indexOf(ext+",") >= 0
};

var lastfieldname = ""
function preview(i) {
 //get current input preview
 var file = i.value

 if (file.length<=0) return;

 //or get get preview for one of form field
  //var file = file_upload.Sourcefile.value
  var ipreview = document.all("ipreview")
 if (isImage(file)) {
  //Show preview for the image.
  ipreview.src = "file://" + file
  //alert(ipreview.fileSize);
  eval("file_upload.fileSize1.value=ipreview.fileSize/1024;");
  //ImageName.innerHTML = "图片预览
(" + i.name + ")"
  if (ipreview.fileSize>10240){
  alert("您选择的图片尺寸大于"+Math.round(10240/1024)+"KB，不能上传");
  //myddd.disabled=true;
  }
  
 } else {
  //some default image for preview
  ipreview.src = "res://shdoclc.dll/warning.gif"
  
  if (lastfieldname != i.name) alert("请选择图片文件 (.gif,.jpg,.png)");
 };
 lastfieldname = i.name
}

window.onerror = donotmsgboxes;
function donotmsgboxes(msg,url,line)
{ // we do not need error messages
 return true
}

</script>

本文来源：http://www.gdgbn.com/wangyezhizuo/17380/