scripting - Inno Setup - File Copy -

I am using Inno Setup with InnoTools downloader and after completing the download I copy selected file to selected directory Want to do

  If CurStep = ssPostInstall then start FileCopy ('Test.exe', ExpandConstant ('{app} \ Test.exe'), false);  

It does nothing, but if I restart the installer and I reinstall it in the same folder, then copy this file. How is this possible or what am I doing wrong? If I do this, then it works well every time:

  If CurStep = ssPostInstall then start FileCopy ('Test.exe', 'Test1.exe', False);    
 
 
 I used the  {src]  solution with using constans Done: 
  add the file itd_addfile ('http://test.com/Test.exe', 'Expand Constant' ('{src} \ Test.exe')); // When the file has finished downloading, copy the file (ExpandConstant ('{src} \ Test.exe'), ExpandConstant ('{app} \ Test.exe'), False); // Delete old file DeleteFile (ExpandConstant ('{src} \ Test.exe'));