[ticket/12526] Fix undefined index: filesize when editing PM with attachments.

PHPBB3-12526
Merge pull request #2439 from prototech/ticket/12526

[ticket/12526] Fix undefined index: filesize when editing PM with attachments.

* prototech/ticket/12526:
  [ticket/12526] Fix undefined index: filesize when editing PM with attachments.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12526] Fix undefined index: filesize when editing PM with attachments.