function Download(const url: String; fnLocal: TFileName): Boolean;
var
  fs: TStream;
  s: TStrings;
  ftp: TIdFTP;
begin
  s := TStringList.Create;
  if CrackUrl(url, s) and (AnsiIndexText(s.Values['scheme'], ['ftp', '']) >= 0) then
  begin
    ftp := TIdFTP.Create(Application);
    ftp.Host := s.Values['hostname'];
    if ftp.Host = '' then ftp.Host := 'localhost';
    ftp.Username := s.Values['username'];
    if ftp.Username = '' then ftp.Username := 'anonymous';
    ftp.Password := s.Values['password'];
    try
      ftp.Connect;
      if FileExists(fnLocal)
        then fs := TFileStream.Create(fnLocal, fmOpenReadWrite or fmShareDenyWrite)
        else fs := TFileStream.Create(fnLocal, fmCreate);
      if ftp.ResumeSupported then
        fs.Position := fs.Size;
      try
        ftp.Get(s.Values['urlpath'], fs, fs.Position > 0);
        Result := True;
      finally
        fs.Free;
      end;
      ftp.Disconnect;
    except
      Result := False;
    end;
  end else Result := False;
  s.Free;
end;

// Getestet habe ich so:

procedure TDemoForm.DownloadButtonClick(Sender: TObject);
const
  fn = 'c:\temp\demo.uxp';
  url = 'ftp://localhost/demo.uxp';
begin
  if FileExists(fn) then
    with TFileStream.Create(fn, fmOpenReadWrite or fmShareDenyWrite) do
    begin
      Size := Size shr 1;
      ShowMessage(Format('size before download: %d', [Size]));
      Free;
    end;

  if Download(url, fn) then
    with TFileStream.Create(fn, fmOpenRead or fmShareDenyNone) do
    begin
      ShowMessage(Format('size after download: %d', [Size]));
      Free;
    end;
end;