I've just had this happen over and over again
Basically possibly the same as: https://tracker.phpbb.com/browse/PHPBB3-9789

To reproduce:

• I went to a members post.
• Clicked edit.
• Added an attachment.
• Submitted the post.
• Viewed the post. The attachment shows fine in the post.
• Now edit the post again, and the attachment is not showing! It's not even showing in the preview section!
• Click 'Attachments' at the bottom and it's not listed there either.
• So I now can't remove the attachment!

• Just gone to index page, gone back to the topic, clicked 'Edit' and now the attachment shows.
• Deleted the attachment
• View post and click 'Edit' again
• And now the ERROR has appeared

***************************************************
General Error
You are not allowed to access this file.

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/includes/message_parser.php
LINE: 1688
CALL: trigger_error()

FILE: [ROOT]/posting.php
LINE: 548
CALL: parse_message->get_submitted_attachment_data()
***************************************************