custom.cs

public static class TextControlExtensions
{
    public static void CustomDialog (
        this TXTextControl.ImageCollection images,
        string preferredFormat = null,
        bool showAll = true,
        OpenFileDialog openFileDialog = null) 
    {
        // create a new open file dialog or use the given one
        OpenFileDialog dlg = (openFileDialog == null) 
            ? new OpenFileDialog() : openFileDialog;

        // retrieve supported filters from Text Control
        string sImportFormats = images.ImportFilters;

        // add an "All Supported Formats" entry with all extensions
        if (showAll) {
            var sAllImportFormats = String.Join(";",
                images.ImportFilters.Split('|')
                    .Where((value, index) => index % 2 == 1)
                    .ToArray());

            sImportFormats = "All Supported Formats|" +
                sAllImportFormats + "|" + sImportFormats;
        }

        // set the filters for the dialog
        dlg.Filter = sImportFormats;

        // select the pre-selected filter
        if (preferredFormat != null) {
            var saFinalFilters = dlg.Filter.Split('|').Where(
                (value, index) => index % 2 == 1).ToArray();

            // set the index (0-based)
            dlg.FilterIndex = Array.FindIndex(saFinalFilters, m => m == preferredFormat) + 1;
        }

        // if file is opened by user, create a new image and insert it
        if (dlg.ShowDialog() == true) {
            TXTextControl.Image image = new TXTextControl.Image(
                System.Drawing.Image.FromFile(dlg.FileName));

            images.Add(image, -1); // at input position
        }
    }
}