C#文件拖放至窗体的ListView控件获取文件类型

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace FileStyle
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public void Data_List(ListView LV, string F) 
        {
            string Format = "";
            if (F.LastIndexOf(".") == F.Length - 4)
                Format = F.Substring(F.LastIndexOf(".") + 1, 3);
            ListViewItem item = new ListViewItem(F);
            item.SubItems.Add(Format);
            LV.It