ACP/Posting/Orphaned attachments, set POST ID and check "Attach file to post" checkbox and submit. File will be attached to post, but board statistics about attachments ('upload_dir_size' and 'num_files' values in CONFIG_TABLE) won't be updated.
The result is when you delete such file from post, you will get a wrong or even negative values in ACP board statistics rows Number of attachments: and Size of posted attachments:.