When extensions for known file types are hidden, an adversary can more easily use social engineering techniques to convince users to execute malicious email attachments. For example, a file named vulnerability_assessment.pdf.exe could appear as vulnerability_assessment.pdf to a user. To reduce this risk, hiding extensions for known file types should be disabled. Showing extensions for all known file types, in combination with user education and awareness of dangerous email attachment file types, can help reduce the risk of users executing malicious email attachments.
The following registry entry can be implemented using Group Policy preferences to prevent extensions for known file types from being hidden.
Taken from : https://www.cyber.gov.au/acsc/view-all-content/publications/hardening-microsoft-windows-10-version-21h1-workstations