Image Upload control in JavaScript

JavaScript Code:

<script language="javascript" type="text/javascript">

        var maxWidth=75;

        var maxHeight=20;

        var fileTypes = ["gif", "png", "jpg", "bmp", "jpeg"];

        var outImage="previewField";

        var defaultPic = "Sunset.gif";

        function preview(what){

          var sourcefile=what.value;

          var ext = sourcefile.substring(sourcefile.lastIndexOf(".") + 1, sourcefile.length).toLowerCase();

          for (var i=0; i<fileTypes.length; i++) if (fileTypes[i]==ext) break;

          globalPic=new Image();

          if (i < fileTypes.length) {

              globalPic.src = sourcefile;

              alert("This is valid image!");

          }

          else {

              globalPic.src = defaultPic;

              alert("That is not avalid image!");

          }

          setTimeout("applyChanged()",500);

        }

        var globalPic;

        function applyChanged(){

          var field=document.getElementById(outImage);

          var x=parseInt(globalPic.width);

          var y=parseInt(globalPic.height);

          if (x>maxWidth) {

            y*=maxWidth/x;

            x=maxWidth;

          }

          if (y>maxHeight) {

            x*=maxHeight/y;

            y=maxHeight;

          }

          field.style.display=(x<1 || y<1)?"none":"";

          field.src=globalPic.src;

          field.width=x;

          field.height=y;

}

</script>

HTML Code:

<input type="file" id="imageField" onchange="preview(this)">