Some of the files for the recently uploaded addins have gone missing. I'm aware of this and I've asked the authors to send them to me, so no need to report missing files for now.

Now to hunt down the source of this bug (which is odd, because I never actually delete files from disk...). I'll post again when this is fixed.

Update 29 March: I found this buried in the Drupal documentation. So now it's fixed permanently.

"The file will be added to the files table as a temporary file. Temporary files are periodically cleaned."