Quick Look Preview is tiny and "It's not know if this application can open <md file>"

[HarmonicDisturbance]

I installed QLMarkdown via brew and have opened it. I've run into the following issues:

1. QLMarkdown does not appear in the Mac's Quick Look Extensions list
2. If I press spacebar on an MD file, the preview is TINY (I suspect this may be either the system or VS Code generating a preview)
3. If I right click and choose "Open with..." on an MD file, QLMarkdown does not show up as a recommended app
4. If I choose "All Applications" and select QLMarkdown, I see the message "It's not know if this application can open <md file>"
5. If I associate a particular MD file with QLMarkdown (via Open with... > Other... > Select QLMarkdown and check Always open with), and then doubleclick on the MD file, I get the same system security warning as if I were opening QLMarkdown for the very first time, except OS X names the MD file and not QLMarkdown as the issue: "Apple could not verify "file.md" is free of malware that may harm..."

Note VS Code is the default app for MD files and will remain so. I have also tried moving QLMarkdown to trash and back but it has not helped.

[sbarex]

QLMarkdown in not an editor for md files, so is not in the open with… menu.
Does the application open when you double-click it?

[HarmonicDisturbance]

The application opens when I double-click it, but the previews when I press the space bar on an MD file are as mentioned tiny (and possibly not generated by QLMarkdown). Finally it does not show up in the Mac's Quick Look Extensions list.

[sbarex]

If the app is not discovered on the System Settings > General & Login Items & Extension / Quick Look, try to move the app on the trash, then back on the Applications folder. And relaunch the app.

[HarmonicDisturbance]

Already tried that...

[soreavis]

Points 3 and 4 ("Open with…" / "not known if this app can open…") are expected — as noted above, QLMarkdown is a Quick Look generator, not an editor, so it intentionally isn't offered in the "Open with" list. Those are separate from the real problem.

The actual issue is that the Quick Look extension isn't getting registered, and the "tiny" preview is the system's plain-text fallback stepping in because QLMarkdown's extension isn't active. Your point 5 is the likely cause: the "Apple could not verify … is free of malware" warning means Gatekeeper is treating the app as quarantined, and a quarantined app's extension won't be loaded — which would explain it being missing from the Quick Look list.

A few things to check beyond the trash-and-back trick:

1. See whether the extension is actually registered:

   pluginkit -m -p com.apple.quicklook.preview | grep -i qlmarkdown
   

   If org.sbarex.QLMarkdown.QLExtension doesn't appear, it isn't registered.

2. Clear the Gatekeeper block behind the malware warning — that's the most likely reason the extension won't load. The simplest way is in System Settings › Privacy & Security › Security (the section at the bottom): after the "could not verify…" warning, an Open Anyway / Allow Anyway button shows up there for the blocked app — confirming it clears the block. I hit the same thing and fixed it that way over on Stoped working on MacOS 27 #207. The Terminal equivalent, if you prefer:

   xattr -l /Applications/QLMarkdown.app          # look for com.apple.quarantine
   xattr -dr com.apple.quarantine /Applications/QLMarkdown.app
   

3. Relaunch the app once, then reset Quick Look and re-check step 1:

   qlmanage -r
   

   If the extension now appears, the spacebar preview should switch from the tiny text fallback to the rendered QLMarkdown view.

If it still doesn't register after clearing quarantine, force-registering the extension directly sometimes does it:

pluginkit -a "/Applications/QLMarkdown.app/Contents/PlugIns/Markdown QL Extension.appex"

Given the malware warning in your point 5, clearing the quarantine attribute (step 2) is the single most likely fix for items 1, 2 and 5 all at once.

This is the same "extension doesn't show up / wrong preview" class as #122 (registration) and #207 (the Gatekeeper security prompt), in case the discussion there helps too.