[ticket/12884] Add core event to the function upload_attachment()

Add core event to the function upload_attachment()
in functions_posting.php to provide ability for extensions
to perform additional actions on uploaded files, like resizing etc.

PHPBB3-12884
Merge pull request #2769 from rxu/ticket/12884

[ticket/12884] Add core event to the function upload_attachment()

* rxu/ticket/12884:
  [ticket/12884] Add core event to the function upload_attachment()