How can I get File Type Text

using System;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace GUITester
    {
    public partial class Form1 : Form
        {
        [StructLayout(LayoutKind.Sequential)]
        public struct SHFILEINFO
            {
            public IntPtr hIcon;
            public IntPtr iIcon;
            public uint dwAttributes;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
            public string szDisplayName;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
            public string szTypeName;
            };

        class Win32
            {
            public const uint SHGFI_DISPLAYNAME = 0x00000200;
            public const uint SHGFI_TYPENAME = 0x400;
            public const uint SHGFI_ICON = 0x100;
            public const uint SHGFI_LARGEICON = 0x0; // 'Large icon
            public const uint SHGFI_SMALLICON = 0x1; // 'Small icon

            [DllImport("shell32.dll")]
            public static extern IntPtr SHGetFileInfo(string pszPath, uint
            dwFileAttributes, ref SHFILEINFO psfi, uint cbSizeFileInfo, uint uFlags);
            }
        public Form1()
            {
            InitializeComponent();
            SHFILEINFO shinfo = new SHFILEINFO();
            IntPtr i = Win32.SHGetFileInfo(@"F:\Temp\YYY.pdf", 0, ref shinfo, (uint) Marshal.SizeOf(shinfo), Win32.SHGFI_TYPENAME);
            string s = Convert.ToString(shinfo.szTypeName.Trim());
            MessageBox.Show(s);
            }
        }
    }