BeforeUpload Event removes ImageUploader Control

    <aur:ImageUploaderFlash ID="ImageUploaderFlash1" runat="server" Height="200px" Width="100%"
        OnFileUploaded="FileUploaded" OnAllFilesUploaded="AllFilesUploaded"
        LicenseKey="<%$ AppSettings:ImageUploaderLicenseKey %>" >
        <ClientEvents>
            <aur:ClientEvent EventName="BeforeUpload" HandlerName="onBeforeUpload" />
        </ClientEvents>
        <Converters>
            <aur:Converter Mode="*.*=Thumbnail" ThumbnailWidth="1600" ThumbnailHeight="1600" />
        </Converters>
        <Restrictions FileMask="[['Images (*.jpg;*.jpeg;*.png;*.gif;*.bmp)', '*.jpg;*.jpeg;*.png;*.gif;*.bmp']]" />
    </aur:ImageUploaderFlash>
    <asp:CheckBox id="CheckBox_AddToDay" text="Also add to day" runat="server" onclick="check(this);" style="float:right"/>
    <script type="text/javascript">
        var addtoday = false;
        function check(cb) {
            //document.getElementById('<%=CheckBox_AddToDay.ClientID %>').checked = cb.checked;
            //<%:addtoday %> = cb.checked;
            //PageMethods.Check_Clicked();
            addtoday = cb.checked;
            alert(cb.checked);
        }
        function onBeforeUpload() {
            // Get folder name
            alert(addtoday);

            // Add folder name field
            this.metadata().addCustomField('addto', addtoday);
        }
    </script>