C#

System.Drawing.Icon.ExtractAssociatedIcon(string filePath)

public Form1()
    {
        InitializeComponent();
        richTextBox1.AllowDrop = true;
        //richTextBox1.EnableAutoDragDrop = true;
        this.richTextBox1.DragEnter += new DragEventHandler(richTextBox1_DragEnter);
        this.richTextBox1.DragDrop += new DragEventHandler(richTextBox1_DragDrop);
    }

    void richTextBox1_DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
    {
        if ((e.Data.GetDataPresent(DataFormats.FileDrop)))
        {
            e.Effect = DragDropEffects.Copy;
        }
    }
    void richTextBox1_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
    {
        //Image img = default(Image);
        //img = Image.FromFile(((Array)e.Data.GetData(DataFormats.FileDrop)).GetValue(0).ToString());
        //Clipboard.SetImage(img);

        //this.richTextBox1.SelectionStart = 0;
        //richTextBox1.AppendText("Hello");
        //this.richTextBox1.Paste();

        /*
            Dim img As Image = Image.FromFile(fname)
            Clipboard.SetImage(img)
            RichTextBox1.Paste()
        */

        string strFilePath = ((Array)e.Data.GetData(DataFormats.FileDrop)).GetValue(0).ToString();
        Icon oIcon= System.Drawing.Icon.ExtractAssociatedIcon(strFilePath);
        Bitmap myBitmap = new Bitmap(oIcon.ToBitmap());
        Clipboard.SetDataObject(myBitmap);
        DataFormats.Format myFormat = DataFormats.GetFormat(DataFormats.Bitmap);
        richTextBox1.Text = "";
        if (richTextBox1.CanPaste(myFormat))
        {
            richTextBox1.Paste(myFormat);
            richTextBox1.AppendText("   " + System.IO.Path.GetFileName(strFilePath));
        }
        else
        {
            MessageBox.Show("The data format that you attempted site" +
              " is not supportedby this control.");
        }
    }