Once you have installed the Attachments::Filesystem plugin, configured it and enabled it, and set Store Attachments, then you would migrate your attachments to the filesystem. To do this you launch a command prompt, change to your root osTicket dir and run the command:
php setup/cli/manage.php file migrate --backend D --to F
If you have a metric ton of attachments you might need to limit how many files it migrates at a time. You can add a limit to the above command line by adding
limit=100 to the end. Which of course would mean that you will need to run the command multiple times to complete the migration from the database to the filesystem.
php setup/cli/manage.php file migrate --backend D --to F limit=100