Did you enable the plugin?
Did you change where osTicket saves the attachments?
Admin panel -> Settings -> Systems see Store Attachments:.
Did you migrate your attachments from the database to the filesystem?
You do this by running a command manually at the command line.
php setup/cli/manage.php file migrate --backend D --to F --limit=100
note: --limit=100 limits it to doing only 100 attachments at a time. You can try running it without the limit, but generally it's a decent idea to test it first with a smaller run.