If I try to preview a HTML file I get:
This file is too big to preview.
Max file size: 50KB
I don't think any of the HTML files I have are less than 100KB and most are much larger. I haven't found a way to increase the limit and I would up to a few MB if I did.
Which viewer is involved? It should say at the top of the preview pane, in brackets after the filename. (Make the window wider, if needed to see the whole thing.)
They all have (Monaco) tacked on to the ends of their file names.
It's odd because I don't know what that is and the default web browser is Firefox and I also have Microsoft Edge.
I just found a few <50 KB files and when I clicked on them they displayed as raw HTML.
I found a few MHTML files and the icon says they are registered to Microsoft Edge and they show just fine.
That's part of Microsoft PowerToys, and causes a lot of problems, as do some of the other viewers in the same bundle.
You can turn it off system-wide in the PowerToys settings. Or via Opus's Preview+Office+Web viewer plugin, if you only want to disable it in Opus.
I'm incredibly impressed you caught that. Even after you mentioned the viewer I didn't find anything when I knew to google for fir the name Monaco.
BTW: Powertoys Settings > File Explorer Add-ons > Source code files (Monaco)
I had to search through the settings for it and only found it because it mentions Monaco, whatever that is.
I just had an epiphany. I mainly installed Powertoys to try out PowerRename and Dopus has completely taken over my my renaming chores. I'm going to see if I need it.