Используйте либо
using System.Drawing;
либо
listView1.SmallImageList.Images.Add(System.Drawing.Icon.ExtractAssociatedIcon(file.FullName));
listView1.LargeImageList.Images.Add(System.Drawing.Icon.ExtractAssociatedIcon(file.FullName));
В результате:
public partial class Form1
{
private void FolderAndFile(DirectoryInfo root)
{
listView1.SmallImageList = new System.Windows.Forms.ImageList();
listView1.LargeImageList = new System.Windows.Forms.ImageList();
foreach (FileInfo file in root.GetFiles())
{
//...
listView1.SmallImageList.Images.Add(System.Drawing.Icon.ExtractAssociatedIcon(file.FullName));
listView1.LargeImageList.Images.Add(System.Drawing.Icon.ExtractAssociatedIcon(file.FullName));
//...
}
}
}