If you get a "MySQL server has gone away" when using the file download feature in vBulletin, this probably means that your forum's attachments are stored in the database.

This increases the size of the database which results in slow MySQL queries that eventually lead to the above error message.

To resolve the issue, you should move your attachments from your database to the file system. You can do this from your vBulletin admin panel > Attachments > Attachment Storage Type.

To move the attachments from the database to the file system, simply click the Go button. This should resolve your problem.