Virus scanning of uploaded attachments

The Attachment Checker allows all attachments to be checked with an external anti-virus scanner when they are uploaded.

This addresses the feature request raised in https://jira.atlassian.com/browse/JRA-8626 (Virus Scan Attachment)

It also solves the performance issue listed in Anti-virus in jira applications where Jira users experience slowness due to scanning of temporary files.

When an attachment is added, a request is sent to scan the uploaded file. The request is handled asynchronously so that users need not wait for the scanning to completed especially if the file is large. 

When a file is suspected to be infected, it will post a comment to the issue in the format "Filename" - "Alert Message".

The user will receive a Jira email notification of the comment to follow up.

Please refer to Additional steps for anti-virus scanning for detailed instructions.