Microsoft has made a security enhancement to Windows to protect you from potential risks when handling files from the Internet. This change affects how preview works in File Explorer. Here's what has changed and what it means for you.

What's changed?
Windows has turned off automatic preview of files downloaded from the internet. This applies in File Explorer when you select a file.

Why is this being done?
To increase safety. Previewing unsafe files can pose risks, such as sensitive information being leaked.

How does it affect me?
When you try to preview a downloaded file, you'll see a warning.
You can still open the file if you trust the source.

Do I need to do anything?
No, you don't need to change anything. It is an automatic security enhancement.

Can I turn preview back on?
Yes, if you're sure the file is safe:
Right-click the file and select Properties.
Click Unblock. 

Read the full article here:
File Explorer automatically disables the preview feature for files downloaded from the internet - M…