The javascript makes the input field to be null and cannot pass variable to

Question

Hi
I use the following code to check whether the upload file is in the right format extension.
However, once I use it and it cannot pass the variable to servlet and throw nullpointerexception.
How can I fix it?
<script language="JavaScript">
    extArray = new Array(".jpg", ".png", ".gif");
    function LimitAttach(form, file) {
    allowSubmit = false;
    if (!file) return;
    while (file.indexOf("\\") != -1)
    file = file.slice(file.indexOf("\\") + 1);
    ext = file.slice(file.indexOf(".")).toLowerCase();
    for (var i = 0; i < extArray.length; i++) {
    if (extArray[i] == ext) { allowSubmit = true; break; }
    if (allowSubmit) return true;
    else
    alert("Please only upload files that end in types:  "
    + (extArray.join("  ")) + "\nPlease select a new "
    + "file to upload and submit again.");
    return false;
</script>
OnSubmit="return LimitAttach(this.form, this.form.myimage.value)"
servlet:
boolean isMultipart = ServletFileUpload.isMultipartContent(request);

Answer 1

the js works fine, just add id=myimage and it is ok
checking file type for uploading only image on client side