[ticket/13108] Add core event to the parse_attachments() function

Add core event to the parse_attachments() function in functions_content.php
to allow modifying/adding attachment template data.

PHPBB3-13108
Merge pull request #2999 from rxu/ticket/13108

[ticket/13108] Add core event to the parse_attachments() function
Merge branch 'develop-ascraeus' into develop