on opening a file upload dialouge box for image only image file should appear in file upload dialouge box

Hi,

<asp:FileUpload ID="FileUpload1" runat="server" />

<asp:Button ID="btnUpload" runat="server" Text="Upload" />

<script type="text/javascript">

    function validateFileUpload(obj){
        var fileName = new String();
        var fileExtension = new String();

        // store the file name into the variable
        fileName = obj.value;

        // extract and store the file extension into another variable
        fileExtension = fileName.substr(fileName.length - 3, 3);

        // array of allowed file type extensions
        var validFileExtensions = new Array("jpg", "png", "gif");

        var flag = false;

        // loop over the valid file extensions to compare them with uploaded file
        for(var index = 0; index < validFileExtensions.length; index++){
            if(fileExtension.toLowerCase() == validFileExtensions[index].toString().toLowerCase()){
                flag = true;
            }
        }

        // display the alert message box according to the flag value
        if(flag == false){
            alert('Files with extension ".' + fileExtension.toUpperCase() + '" are not allowed.\n\nYou can upload the files with following extensions only:\n.jpg\n.png\n.gif\n');
            return false;
        }
        else{
            alert('File has valid extension.');
            return true;
        }
    }
</script>

protected void Page_Load(object sender, EventArgs e)
{
    FileUpload1.Attributes.Add("onchange", "return validateFileUpload(this);");

    btnUpload.Attributes.Add("onclick", "return validateFileUpload(document.getElementById('" + FileUpload1.ClientID + "'));");
}