Пример ниже достает описание тектового файла.

using System;
using System.Runtime.InteropServices;
using System.ComponentModel;

[StructLayout(LayoutKind.Sequential)] 
public struct SHFILEINFO 
{ 
  public IntPtr hIcon; 
  public int iIcon; 
  public uint dwAttributes; 
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)] 
  public string szDisplayName; 
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)] 
  public string szTypeName; 
} 


public class TestClass 
{ 
  public const uint SHGFI_TYPENAME = 0x000000400; 
  public const uint SHGFI_USEFILEATTRIBUTES = 0x000000010; 

  [DllImport("Shell32.dll")] 
  public static extern IntPtr SHGetFileInfo( 
    string pszPath, 
    uint dwFileAttributes, 
    ref SHFILEINFO psfi, 
    uint cbFileInfo, 
    uint uFlags); 

  [STAThread]
  public static void Main() 
  { 
    SHFILEINFO shfi = new SHFILEINFO(); 
    SHGetFileInfo("*.txt", 0, ref shfi, (uint)Marshal.SizeOf(shfi), SHGFI_TYPENAME | SHGFI_USEFILEATTRIBUTES); 
    Console.WriteLine(shfi.szTypeName); 
    Console.ReadLine(); 
  } 
} 

По топику http://www.dotsite.ru/Forums/8992.aspx