Please refer below article

Upload files, save in folder and display in ASP.Net GridView with Download and Delete option

also refer below code. to access files from muliple folder.

C#

string[] filePaths = new string[] { };
filePaths = Directory.GetFiles(Server.MapPath("~/Uploads/"));
filePaths = Directory.GetFiles(Server.MapPath("~/Files/"));
List<ListItem> files = new List<ListItem>();
foreach (string filePath in filePaths)
{
    files.Add(new ListItem(Path.GetFileName(filePath), filePath));
}
GridView1.DataSource = files;
GridView1.DataBind();