I have custom mail handler which creates Jira issue and then makes an attachment from message and add it to that issue
attachmentManager.createAttachment(File, String, String, User, Issue);
It works I have new issue with attachment.
Then I try to make webhook with Issue Related Events

But webhooks works only on issue created and sends json with empty attachments
{"webhookevent":"jira:issue_created" "id":"123","key":"ABC", "fields":{"project"{"id", "key", "name"}, "attachment":[]}}