From the customer support case

When using os_authType=basic to login to JIRA 4.2 a user is able to upload an attachment as a temporary file, but is unable to attach the temporary file to the issue. We noticed the exact same behavior [...] had worked with JIRA 4.1.2.

The Atlassian support people have confirmed this as a bug in 4.2. I suspect at this stage its perhaps related to SER-154.

To re-create :

• Restart your browser each time (basic auth creds are cached by the browser
• Goto jira with ?os_authType=basic to cause a basic auth challenge
• Click Create Issue
• Attach a file
• See the error "Temporary attachment with id '8,995,549,407,744,639,063' not found. Session may have timed out before submitting the form."

It appears the "Attach Files" on view issue is also broken. I managed to get one to work and then a second to fail. I am still investigating this aspect.