[ticket/13046] Call phpbb_load_extensions_autoloaders() in $download/file.php

PHPBB3-13046
Merge pull request #2928 from Nicofuma/ticket/13046

[ticket/13046] Call phpbb_load_extensions_autoloaders() in $download/file.php

* Nicofuma/ticket/13046:
  [ticket/13046] Call phpbb_load_extensions_autoloaders() in $download/file.php