Validate File upload Control

function ValidateFileUpload(ControlID, filename) {

            var isValid = false;

            var Extension = filename.substring(filename.lastIndexOf('.') + 1).toLowerCase();

            var extArray = new Array("gif", "jpg", "tif", "jpeg", "tiff", "pdf", "txt", "doc", "docx", "rtf");

            for (var i = 0; i < extArray.length; i++) {

                if (extArray[i] == Extension) {

                    isValid = true;

                    break;

                }

            }

            if (isValid == false) {

                document.getElementById(ControlID).value = "";

                alert(' <%=ViewRes.Common.Invalidfileformat%>');

            }

        }

<input class="txtfld FileUpload" onchange="ValidateFileUpload(this.id,this.value)"

                            type="file" id="file1" name="AttributeImages1" />