Coming soon for Microsoft Purview | Data Loss Prevention for Microsoft Exchange Online:

Before this rollout, Exchange Online relies only on the file extension (such as .docx and .pdf) to identify the type of an attachment but this approach can be bypassed if a file is renamed with a misleading extension. To address this, we’re enhancing our detection logic to go beyond just the visible file extension. After this rollout, even if a file’s extension is altered, Exchange will evaluate the actual content of the file to determine its true type. This ensures that policies based on file type (such as File extension is…) will remain effective and secure, even when attachments are renamed or disguised. This improvement provides you with a more robust and reliable way to enforce compliance and security rules around file sharing.

This message is associated with Microsoft 365 Roadmap ID 493291.