'Invalid utf8 character string' warning when importing into AWS RDS MySQL
Judging by your initial post, where you said you used default options for mysqldump, it means you didn't use the --hex-blob option. I've played around with osTicket some time ago, and you should definitely used --hex-blob option to create your dump. It will give you much bigger .sql file (and of course, the import to RDS will take more time) but in that way you won't get the UTF8 warnings you're getting now, and your attachments will be available once you deploy osTicket to new host.

One more advice, update to newer versions (1.9 or 1.10) and move those files out of DB, you can use S3 or even EFS now, or just store it under your EBS volume