function checkFileExtension(elem) {
        var filePath = elem.value;
 
        if(filePath.indexOf('.') == -1)
            return false;
 
        var validExtensions = new Array();
        var ext = filePath.substring(filePath.lastIndexOf('.') + 1).toLowerCase();
        //Add valid extentions in this array
        validExtensions[0] = 'doc';
        //validExtensions[1] = 'pdf';
 
        for(var i = 0; i < validExtensions.length; i++) {
            if(ext == validExtensions[i])
                return true;
        }
 
        alert('The file extension ' + ext.toUpperCase() + ' is not allowed!');
        return false;
    }

Public Shared Function GetFileName(ByVal Path As String) As String
        Try
            Dim RetVal As String
            Dim dt As String
            dt = Format(Date.Today, "yyMMdd")
            RetVal = Path & "\" & dt & ".xls"
            'RetVal = Path & "\" & "test.BIL"
            If File.Exists(RetVal) = True Then
                Return RetVal
            Else
                RetVal = Nothing
            End If
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Function