Download Files from GridView placed inside AJAX UpdatePanel in ASP.Net

Here I have created sample that will help you out.

Master Page

<div>
    *********Master Page:*************
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" EmptyDataText="No files uploaded">
        <Columns>
            <asp:BoundField DataField="Text" HeaderText="File Name" />
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:LinkButton ID="lnkDownload" Text="Download" CommandArgument='<%# Eval("Value") %>'
                        runat="server" OnClick="DownloadFile"></asp:LinkButton>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <hr />
    <div>
        <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
        </asp:ContentPlaceHolder>
    </div>
</div>

Master Page Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string[] filePaths = Directory.GetFiles(Server.MapPath("~/Uploads/"));
        List<ListItem> files = new List<ListItem>();
        foreach (string filePath in filePaths)
        {
            files.Add(new ListItem(Path.GetFileName(filePath), filePath));
        }
        GridView1.DataSource = files;
        GridView1.DataBind();
    }
}

protected void DownloadFile(object sender, EventArgs e)
{
    string filePath = (sender as LinkButton).CommandArgument;
    Response.ContentType = Page.ContentType;
    Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(filePath));
    Response.WriteFile(filePath);
    Response.End();
}

Default Page

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    ***********Content Page:*************
</asp:Content>

Screenshot

Hi EmadKhan,

Use Below Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string[] filePaths = Directory.GetFiles(Server.MapPath("~/Uploads/"));
        List<ListItem> files = new List<ListItem>();
        foreach (string filePath in filePaths)
        {
            files.Add(new ListItem(Path.GetFileName(filePath), filePath));
        }
        GridView1.DataSource = files;
        GridView1.DataBind();
    }
    ScriptManager scriptManager = ScriptManager.GetCurrent(this.Page);
    scriptManager.RegisterPostBackControl(this.GridView1);
}

I hope this will help you out.