ASPX Code

<ext:GridPanel ID="grdDownload" runat="server" StoreID="StoreDownload" StripeRows="true"
Height="430" Width="600px" Header="true" AutoShow="False" AutoScroll="True" >
<ColumnModel ID="ColumnModel1" runat="server">
<Columns>
<ext:GroupingSummaryColumn ColumnID="CLATeamNo" Header="" Width="150" Sortable="true"
DataIndex="CategoryName">
</ext:GroupingSummaryColumn>
<ext:GroupingSummaryColumn Header="" Width="130" Sortable="true" DataIndex="DisplayName">
</ext:GroupingSummaryColumn>
<ext:GroupingSummaryColumn Header="" Width="130" Sortable="true" DataIndex="Description">
</ext:GroupingSummaryColumn>
<ext:ImageCommandColumn Width="40px" DataIndex="DocumentUrl" >
<Commands>
<ext:ImageCommand CommandName="OpenFile" Icon="ServerLink">
<ToolTip Text="Open File" />
</ext:ImageCommand>
</Commands>

</ext:ImageCommandColumn>

</Columns>
</ColumnModel>
<View>
<ext:GroupingView ID="GroupingView1" runat="server" ForceFit="true" ShowGroupName="false"
EnableNoGroups="true" HideGroupedColumn="true" />
</View>
<Plugins>
<ext:GroupingSummary ID="GroupingSummary1" runat="server" />
</Plugins>
<SelectionModel>
<ext:CellSelectionModel ID="cellSel" runat="server">
<AjaxEvents>
<CellSelect OnEvent="DwnloadImageItemClickAjax">
<ExtraParams>
<ext:Parameter Name="DownloadItem" Value="DownloadImg" />
</ExtraParams>
<EventMask ShowMask="true" />
</CellSelect>
</AjaxEvents>

</ext:CellSelectionModel>
</SelectionModel>
</ext:GridPanel>


ASPX.CS CodeBehind

[AjaxMethod]
protected void DwnloadImageItemClickAjax(object sender, AjaxEventArgs e)
{

cellSel.AjaxEvents.CellSelect.IsUpload = true;
this.Context.Response.Clear();
this.Context.Response.AddHeader("Content-Disposition", string.Format("attachment; filename=abc.xlsx"));
this.Context.Response.ContentType = "application/octet-stream";
this.Context.Response.WriteFile(string.Format("d:/p/abc.xlsx"));

}

I am getting this error:

Request Failure
Bad Response: Invalid Character