为何在线程中打开文件对话窗就无法跳出？(100分)

unit MainDo;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, IdBaseComponent, IdComponent, IdTCPConnection, IdTCPClient,
  IdExplicitTLSClientServerBase, IdFTP, StdCtrls, ComCtrls;
type
  TupThread = class(TThread)
  private
    { Private declarations }
    procedure Upload;
  protected
    procedure Execute;
override;
  end;

type
  TForm1 = class(TForm)
    FTP: TIdFTP;
    OpenDialog1: TOpenDialog;
    Button1: TButton;
    Button2: TButton;
    ProgressBar1: TProgressBar;
    StatusBar1: TStatusBar;
    procedure FTPWorkEnd(ASender: TObject;
AWorkMode: TWorkMode);
    procedure FTPWorkbegin
(ASender: TObject;
AWorkMode: TWorkMode;
      AWorkCountMax: Integer);
    procedure FTPWork(ASender: TObject;
AWorkMode: TWorkMode;
      AWorkCount: Integer);
    procedure Button2Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
      FTP.Host := '61.155.39.11';
      FTP.Username := 'upload';
      FTP.Password := 'upload';
      FTP.Connect;
      if FTP.Connected then
        begin
           StatusBar1.SimpleText := '成功连接到服务器，请选择一个视频开始上传...';
           Button1.Enabled := false;
           Button2.Enabled := true;
        end
      else
        begin
           StatusBar1.SimpleText := '连接到服务器失败，请稍后重试...';
           Button1.Enabled := true;
           Button2.Enabled := false;
        end;
end;

procedure TupThread.Execute;
begin
  { Place thread code here }
  Upload;
  //Synchronize();
end;

procedure TupThread.Upload;
 begin
      if Form1.FTP.Connected then
      begin
            if Form1.OpenDialog1.Execute then
                begin
                    Form1.FTP.Put(Form1.OpenDialog1.FileName, ExtractFileName(Form1.OpenDialog1.FileName));
                end
            else
                begin
                    showmessage('出现未知错误，请重新启动程序。');
                end;
      end
      else
      begin
         showmessage('尚未连接到服务器，请先连接。');
         Form1.Button1.Enabled := true;
         Form1.Button2.Enabled := false;
      end;
 end;

procedure TForm1.Button2Click(Sender: TObject);
var
  T : TThread;
begin
  T := TupThread.Create(false);
end;

procedure TForm1.FTPWork(ASender: TObject;
AWorkMode: TWorkMode;
  AWorkCount: Integer);
begin
             
       ProgressBar1.Position := AWorkCount;
end;

procedure TForm1.FTPWorkbegin
(ASender: TObject;
AWorkMode: TWorkMode;
  AWorkCountMax: Integer);
begin
                   
      ProgressBar1.Max := AWorkCountMax;
      ProgressBar1.Position := 0;
      ProgressBar1.Visible := true;
end;

procedure TForm1.FTPWorkEnd(ASender: TObject;
AWorkMode: TWorkMode);
begin
      ProgressBar1.Position := ProgressBar1.Max;
end;

end.