[Solved]-Gridview sorting in ASP.NET-LINQ,C#

You can't use the DataSource property of the grid to fetch the data source. It is only available after it is set and to the end of that postback. You'll need the fetch the data from the database again.
Something like this:

public void GridView1_Sorting(object sender, GridViewSortEventArgs e)
{
  var users = (from user in dataContext.tbl_files
              select new { user.File_Name, user.Upload_Time, user.Uploaded_By }).ToList().AsEnumerable();

  switch(e.SortExpression)
  {
     case "File_Name":
       users = users.OrderBy(x => x.File_Name);
       break;
     case "Upload_Time":
       users = users.OrderBy(x => x.Upload_Time);
       break;
     case "Uploaded_By":
       users = users.OrderBy(x => x.Uploaded_By);
       break;
  }
  if(e.SortDirection == SortDirection.Descending)
    users = users.Reverse();

  GridView1.DataSource = users;
  GridView1.DataBind();
}