Later: I now find it doesn't seem to be about attachments as such - I have the same issue on another question without attachments, where I have a support file and the same line is giving the same error relating to that support file, as long as 'validate answer' is ticked. Making the same change to the template also fixes that error. I think what led to this was refreshing the support file (dropping in the latest version).
Forum
Question Authors' Forum
Using more than one JDK / Java prototype
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 5 posting(s) in 1 day