file upoload javascript and jquery validation

/*First you need this in your html */
 <fieldset class="images">
    <input type="file" name="imagenupload1" id="imagenupload1" tabindex="25" />
</fieldset>




/*Now in your javascript file*/
/*First we define some variables*/
var validar_imagenupload=new Boolean(false);
var validar_imagenextension=new Boolean(false);
/* This are the valid extensions. Add or remove as you wish */
var extensiones_permitidas = new Array(".gif", ".jpg", ".jpeg", ".png");


    /* Now, the image upload validation */
    $('#imagenupload1').change(function(){
        validar_imagenupload=false;
        validar_imagenextension = false;
        var node = document.getElementById('imagenupload1');
        var check = node.files[0].fileSize;
        var nombreImg =  node.files[0].fileName;
        var extension = (nombreImg.substring(nombreImg.lastIndexOf("."))).toLowerCase();

        /* check for maximum size. In this case is 300KB. You can set to whatever you want  */
        if (check < 300001){
            //Size is OK
            /* Check file extension */
            for (var i = 0; i < extensiones_permitidas.length; i++) {
                if (extensiones_permitidas[i] == extension) {
                    validar_imagenupload = true;
                    validar_imagenextension = true;
                    break;
                } else{
                    validar_imagenextension = false;
                }
            }

        } else{
            alert('File is too big');
            clearFileUpload('imagenupload1');
            validar_imagenextension = true;
        }
        if (validar_imagenextension == false){
            alert('File is not an image');
            clearFileUpload('imagenupload1');
        }

    });




/* We also need this function */
function clearFileUpload(id){
    // get the file upload element
    fileField = document.getElementById(id);
    // get the file upload parent element
    parentNod = fileField.parentNode;
    // create new element
    tmpForm = document.createElement("form");
    parentNod.replaceChild(tmpForm,fileField);
    tmpForm.appendChild(fileField);
    tmpForm.reset();
    parentNod.replaceChild(fileField,tmpForm);
}