SalesForce Securlet is unable to quarantine attachments that violates policies.
SalesForce Securlet can quarantine files in Salesforce.
Note: (Data stored as files in Salesforce is different than data stored as an attachment).
Salesforce recommends using files instead of attachments because of limitations with attachments.
Recomendation: Use Lightning experience where only files are supported.
Alternatively In Salesforce Classic mode you can convert attachments to files.
Go to Setup.
Go to Salesforce Files (quick find: type “Salesforce Files”)
Click General setting.
Check “Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments”