[ticket/13396] Normalize multibyte characters in attachment file names.

PHPBB3-13396
[ticket/13400] Information text if the server load id high at search

PHPBB3-13400
Merge branch 'ticket/13400' into develop-ascraeus
Merge pull request #3208 from prototech/ticket/13396

[ticket/13396] Normalize multibyte characters in attachment file names.