function XCompareFilename(i1, i2: Integer): Integer;
{ Custom sort function (by Filename). }
var
  s1, s2: string;
begin
  s1 := ExtractFileName(Form1.Collection1.MIO.Params[i1].FileName);
  s2 := ExtractFileName(Form1.Collection1.MIO.Params[i2].FileName);
  if s1 < s2 then
    Result := -1
  else if s1 > s2 then
    Result := 1
  else
    Result := 0;
end;

if ToggleSort1.Down then
  Form1.Collection1.Sort(XCompareFilename)
else
  Form1.Collection1.Sort(iesbNone);
Form1.Collection1.Update;