Cannot Preview Network Files Within File Explorer

This is not a bug in the strixt sense. Microsoft intentionally changed the behavior of the Preview to close a securety gap.

The cleanest and most sustainable fix is to add your network shares to the Local Intranet zone in the legacy Internet Options. Files coming from a zone Windows considers trusted will not get the Internet zone marking, and the Preview Pane will render them normally again. Uninstalling KB5066835 also resolves the issue, but is not recommended because it leaves the system without the October 2025 security patches.

To apply the fix, press Windows + R, type inetcpl.cpl, and press Enter - this opens the Internet Properties dialog. Switch to the Security tab, select the Local intranet icon, and click Sites. In the small dialog that opens, click Advanced. You are now in the dialog where the trusted intranet locations are managed. Important: before adding anything, uncheck the box "Require server verification (https:) for all sites in this zone". This is the step most guides leave out, and without it the fix will silently fail for SMB and UNC paths, because those use neither HTTP nor HTTPS.

Now add your network locations in the field "Add this website to the zone" and click Add after each entry. The format depends on how you access the share. For a server reached by hostname, enter file://servername (for example file://fileserver01). For a server reached by IP address, enter file://192.168.1.10 with the appropriate IP.

If your environment uses a domain like company.local, you can add the whole domain at once with file://*.company.local, which covers every server inside that domain. You can add multiple entries - one per line in the list - to cover all the file servers your users access. When finished, click Close, then OK to leave the Internet Options.

The change takes effect immediately for newly opened Explorer windows, but in some cases Explorer caches the old behavior. If previews still don't appear, restart File Explorer: open Task Manager (Ctrl + Shift + Esc), find Windows Explorer in the process list, right-click it, and choose Restart. A full reboot is not required.

go to 

Open Regedit,

go to : HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Zones\3]

after that search for Key 180F -> Change value to 0

Reboot the machine it should fix the issue

The simplest solution for Windows 11 is:

Uninstall Windows Security Update (KB5066835) under Settings -> Windows Update.

Previews will reappear.

I am also facing this making my work much slower - everytime I have to open the document to see its contents. 

Until recenty, every file preview was visible in the preview pane. Since the last few days, preview of some files are visible; for some, I get an error message -  'The file you are attmpting to preview could harm your computer. If you trust the file and the source you received it from, open it to view the contents'. But, previews of the ones with error message were visible a coupl eof weeks back and now they have become harmful! 

Can someone please help solve this issue with Windows Explorer?