怎么样通过SavePictureDialog控件另存图片

Question

我通过鼠标单击按钮,调用SavePictureDialog控件,保存的图片,没有类型后缀,自己手动加上文件名后缀后,能正常显示图片.procedure TForm1.N3Click(Sender: TObject);
var FromF,ToF:file;
    NumRead,NumWritten:Integer;
    Buf:array [1..2048] of Char;
begin
 SavePictureDialog1.Title:='图片另存为';
 SavePictureDialog1.Execute;
 AssignFile(FromF,filelistbox1.filename);
 Reset(FromF,1);
 AssignFile(ToF,SavePictureDialog1.FileName);//这里只有文件名,不知道后缀怎么加上去
 Rewrite(ToF,1);
 repeat
 BlockRead(FromF,Buf,SizeOf(Buf),NumRead);
 BlockWrite(ToF,Buf,NumRead,NumWritten);
 Until (NumRead=0)or(NumWritten<>NumRead);
 CloseFile(FromF);
 CloseFile(ToF);
end;