Hi itsme,

Check this example. Now please take its reference and correct your code.

For displaying file in new window refer below link.

Open Word Excel PDF PowerPoint (PPT) files in new browser window in ASP.Net

HTML

<asp:FileUpload ID="FileUpload1" runat="server" />
<hr />
<asp:Button ID="btnUpload" Text="Upload" runat="server" OnClick="btnUpload_Click" />
<asp:DropDownList ID="drp" runat="server" AutoPostBack="true" OnSelectedIndexChanged="SelectedIndexChanged">
</asp:DropDownList>

Namespace

C#

using System.IO;

VB.Net

Imports System.IO

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        PopulateFileList();
    }
}

protected void SelectedIndexChanged(object sender, EventArgs e)
{
    Response.Redirect("~/Default.aspx?FileName=" + drp.SelectedValue);
}

private void PopulateFileList()
{
    DirectoryInfo di = new DirectoryInfo(Server.MapPath("~/Files/"));
    FileInfo[] fis = di.GetFiles();
    drp.DataSource = fis;
    drp.DataTextField = "Name";
    drp.DataValueField = "Name";
    drp.DataBind();
    drp.Items.Insert(0, new ListItem("Select", "0"));
}

protected void btnUpload_Click(object sender, EventArgs e)
{
    string folderPath = Server.MapPath("~/Files/");

    //Check whether Directory (Folder) exists.
    if (!Directory.Exists(folderPath))
    {
        //If Directory (Folder) does not exists. Create it.
        Directory.CreateDirectory(folderPath);
    }

    //Save the File to the Directory (Folder).
    FileUpload1.SaveAs(folderPath + Path.GetFileName(FileUpload1.FileName));
    PopulateFileList();
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        PopulateFileList()
    End If
End Sub

Protected Sub SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Response.Redirect("~/Default.aspx?FileName=" & drp.SelectedValue)
End Sub

Private Sub PopulateFileList()
    Dim di As DirectoryInfo = New DirectoryInfo(Server.MapPath("~/Files/"))
    Dim fis As FileInfo() = di.GetFiles()
    drp.DataSource = fis
    drp.DataTextField = "Name"
    drp.DataValueField = "Name"
    drp.DataBind()
    drp.Items.Insert(0, New ListItem("Select", "0"))
End Sub

Protected Sub btnUpload_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim folderPath As String = Server.MapPath("~/Files/")
    If Not Directory.Exists(folderPath) Then
        Directory.CreateDirectory(folderPath)
    End If
    FileUpload1.SaveAs(folderPath & Path.GetFileName(FileUpload1.FileName))
    PopulateFileList()
End Sub

Default Page

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(Request.QueryString["FileName"]))
    {
        string fileName = Request.QueryString["FileName"];
        string filePath = Server.MapPath("~/Files/") + fileName;
        string fileExtension = Path.GetExtension(filePath);
        switch (fileExtension.ToLower())
        {
            case ".txt":
                Response.Write(File.ReadAllText(filePath));
                break;
            case ".pdf":
                Response.ContentType = "Application/pdf";
                Response.WriteFile(filePath);
                Response.End();
                break;
            case ".jpg":
                Response.ContentType = "image/JPEG";
                Response.WriteFile(filePath);
                Response.End();
                break;
            case ".gif":
                Response.ContentType = "image/GIF";
                Response.WriteFile(filePath);
                Response.End();
                break;
            default:
                break;
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not String.IsNullOrEmpty(Request.QueryString("FileName")) Then
        Dim fileName As String = Request.QueryString("FileName")
        Dim filePath As String = Server.MapPath("~/Files/") & fileName
        Dim fileExtension As String = Path.GetExtension(filePath)

        Select Case fileExtension.ToLower()
            Case ".txt"
                Response.Write(File.ReadAllText(filePath))
            Case ".pdf"
                Response.ContentType = "Application/pdf"
                Response.WriteFile(filePath)
                Response.End()
            Case ".jpg"
                Response.ContentType = "image/JPEG"
                Response.WriteFile(filePath)
                Response.End()
            Case ".gif"
                Response.ContentType = "image/GIF"
                Response.WriteFile(filePath)
                Response.End()
            Case Else
        End Select
    End If
End Sub

Screenshot