Email Accounts Not Assigned to any JIRA User Cannot Add Attachments to Tickets Using the Mail Handler

Currently when you Share a JIRA issue with an email account which is not assigned to any specific user the JIRA mail handler will then allow that email address to add comments to the ticket (the comments are then assigned to the default reporter set on the mail hander). But if that email contains an attachment the update will fails with a lack of permissions error.

Steps to Replicate:

1. Create a ticket in JIRA
2. Share that ticket with an email account that is not assigned to any JIRA Users
3. Attempt to update the ticket (by referencing the issue key on the subject) from that email account which initially received the share, without any attachments.
4. Perform the same test, now with attachments.

Step 2 will result in comments added to the ticket, this is what's shown on the logs:

2014-09-24 11:56:44.092096500 2014-09-24 11:56:44,092 atlassian-scheduler-quartz1.clustered_Worker-1 INFO ServiceRunner    TestHandler \[mail.incoming.CreateOrCommentHandler.1] Sender(s) \[example@email.com] not registered in JIRA. Using configured default reporter 'defaultreporter'.
2014-09-24 11:56:44.104649500 2014-09-24 11:56:44,104 atlassian-scheduler-quartz1.clustered_Worker-1 INFO ServiceRunner    Test \[atlassian.mail.incoming.mailfetcherservice] Test[10001]: Added comment 'Test after sharin... 'by 'defaultreporter' to issue 'TEST-1'

Step 3 will result in the following error:

2014-09-24 12:17:10.829007500 2014-09-24 12:17:10,828 atlassian-scheduler-quartz1.clustered_Worker-3 DEBUG ServiceRunner    Comments from emails \[atlassian.mail.incoming.mailfetcherservice] The mail 'FROM' does not match a valid user
2014-09-24 12:17:10.829009500  This user is not in JIRA so can not add a comment: username <example@email.com>

Since JIRA allows comments to be performed by external users who were shared on the ticket it's expected that attachments added via email would work the same. This inconsistent behaviour will cause a lot of problems to users who use JIRA this way, resulting in comment updates received via emails from external customers being lost.