question about the sandboxing

Hello everyone. I want to enable the sandbox feature on my firewall, and my license supports it. However, how can I ensure that my files, which will be uploaded and scanned to the cloud, are secure and won't be accessed by third parties? I want to know that uploading them to the cloud is solely for scanning and then they are removed... Is there any guarantee that my data will remain safe from being accessed by any other parties, including Fortigate?