On access scanning in Attachment Checker is a marker to let the app know if it has been enabled in the 3rd party virus scanner.
For virus scanners with on access scanning enabled, the file may be deleted after it is uploaded into Jira, without any feedback.
Users will only find out when they try to preview or download the attachment, which will redirect them to a dead link.
If on access scanning is enabled, a comment will then be added as a feedback to users that the file has been deleted.
If delete virus files is enabled, the attachment record will also be deleted from the database.
The on access scanning can be enabled at Attachment Checker Configuration → Virus Scanning