string text;
for (int i = 0; i <= 999; i++)
{
    text.Append("*." + i.ToString("000") + "; ");
}

string textWithoutLastSemicolumn = text.ToString().Substring(0, text.ToString().Length - 2);
dialog.Filter = "Files (" + textWithoutLastSemicolumn + ")|" + textWithoutLastSemicolumn;

ofd.Filter = "Supported extensions | *.0??;*.1??;*.2??;*.3??;*.4??;*.5??;*.6??;*.7??;*.8??;*.9??";

StringBuilder sb = new StringBuilder();
for (int i = 0; i <= 999; i++)
{
    sb.Append("*.");
    sb.Append(i.ToString("000"));
    sb.Append("|");
}
sb.Remove(sb.Length - 1, 1);
string text = sb.ToString();

OpenFileDialog ofd = new OpenFileDialog();
ofd.Filter = "Files (" + text + ")|" + text;
ofd.ShowDialog(this);

StringBuilder sb = new StringBuilder();
for (int i = 0; i <= 999; i++)
{
    sb.Append("*.");
    sb.Append(i.ToString("000"));
    sb.Append("|");
}
sb.Remove(sb.Length - 1, 1);
string text = sb.ToString();

OpenFileDialog ofd = new OpenFileDialog();
ofd.Filter = text;
ofd.ShowDialog(this);

dialog.AutoUpgradeEnabled = false;

OpenFileDialog dialog = new OpenFileDialog();
StringBuilder text = new StringBuilder();
for (int i = 0; i <= 999; i++)
{
    text.Append("*." + i.ToString("000") + "; ");
}

string textWithoutLastSemicolumn = text.ToString().Substring(0,    text.ToString().Length - 2);
dialog.AutoUpgradeEnabled = false;
dialog.Filter = "Files (" + textWithoutLastSemicolumn + ")|" + textWithoutLastSemicolumn;
dialog.ShowDialog();