CustomValidator doesn't work correctly in asp.net

I created a custom validator for FileUpload for control photos size and format

protected void cvrFileUpload_ServerValidate(object source, ServerValidateEventArgs args)
    {
        if (rbtnSelectByFile.Checked)
        {
            if (fuplBrows.HasFile)
            {
                string fileType = Path.GetExtension(fuplBrows.PostedFile.FileName).ToLower().Trim();
                if (fileType != ".jpg" && fileType != ".png" && fileType != ".bmp" && fileType != ".jpeg")
                {

                    cvrFileUpload.ToolTip = "Only .jpg, .png, .bmp file formats are allowed";
                    args.IsValid = false;
                    ScriptManager.RegisterStartupScript(this, GetType(), "pagechange", "nextPage(); ", true);
                }
                else
                {
                    if (fuplBrows.PostedFile.ContentLength > 102400)
                    {
                        cvrFileUpload.ToolTip = "حجم فایل باید کمتر از 100 کیلوبایت باشد";
                        args.IsValid = false;
                        ClientScript.RegisterStartupScript(GetType(), "pagechange", "nextPage(); ", true);
                        return;
                    }
                    else
                    {
                        args.IsValid = true;
                    }
                }
            }
        }

    }

And have a button for saving information in the database that must not work if customvalidator is invalid:

protected void btnRegist_Click(object sender, EventArgs e)
        {

            ResultManage oRm = new ResultManage();

            RequestInfo oRi = form2oRi();

            int id = oRm.saveResult(oRi);

            if (id > 0)
            {
                Response.Redirect("~/RecordedResult.aspx");
            }
            else
            {
                ClientScript.RegisterStartupScript(this.GetType(), "پیام سیستم", "alert('خطا در انجام عملیات');", true);
            }

        }

I want to not work my button function when custom validator is invalid. How do I do this?

I'd suggest you use the

Page.IsValid 

Property to check if all Validators are valid:

http://msdn.microsoft.com/en-us/library/system.web.ui.page.isvalid(v=vs.110).aspx