If you view the properties of such types of files, you will see no file extension (see the above screenshot). Here, neither you nor your operating system knows the file extension.

Luckily, it isn’t difficult to enable the setting to view file extensions in both Windows 10 and 11. For Windows 10, open File Explorer, choose “View,” then click the tab next to “File ...

Microsoft hides file extensions in Windows by default even though it's a security risk that is commonly abused by phishing emails and malware distributors to trick people into opening malicious files.

An mp4 is a video file type, but the .mp4 in the file name is the file extension. Usually, these things should match, but it's possible for that not to be the case.

To view the properties of a file, right-click on it and select Properties. In the above screenshot, we have compared the properties of a file with a .pdf extension and a file with no extension ...

As BleepingComputer explains: hackers will send malware files to users via spam, trick browser downloads and more and they often look like innocent files thanks to Windows 10 hiding their extension.

Read about changes to file types blocked in Outlook on the web, which will prevent download of attachments with file extensions associated with Python, PowerShell, digital certificates, Java, etc ...