The old attachments will be in the database. You can migrate them out of the database if you want.
Step 1: install the plugin
Step 2 configure and enable the plugin
Step 3: change where files are stored in osTicket.
go to: Admin panel -> Settings -> System and set store attachments
Step 4: migrate the files from the DB to the FS (File System)
To migratre files open a command prompt and run:
php setup/cli/manage.php file migrate --backend D --to F --limit=20
php needs to be the path to the version of PHP that you are running (if you run more than one).
limit is 20, which means that it will only do 20 files. But you can change the number if you want. We just want to make sure that you dont have a problem with the command before we start doing a bunch of them.