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When a .ts file previews as a video, or not at all

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Press Space on app.ts and Quick Look shows a media player with no picture, a blank panel, or an error about an unreadable movie. The file is TypeScript. macOS is not sure.

Two formats share the extension

.ts has meant “MPEG transport stream” since long before it meant TypeScript. Broadcast recordings, Blu-ray captures, and camera clips use it. When Finder decides which previewer to hand a .ts file to, it goes by the file type macOS has registered for that extension, and on some systems that is the video one. The same applies to .mts, which is both a TypeScript module and an AVCHD camera clip.

That decision happens before Syntax Look is involved. If the system routes the file to a media handler, no code previewer gets asked.

What Syntax Look does when it is asked

When the file does reach Syntax Look, it checks the payload before rendering. A real transport stream is binary, and the extension detects and rejects it safely: you get a clean fallback rather than a screen of garbled bytes. A real TypeScript file renders as code.

Which TypeScript files preview reliably

.tsx and .cts are not shared with any media format, so they preview consistently. If you can choose, those are the safe targets. Plain .ts and .mts depend on which type macOS has registered for your machine, which is a platform-level behavior we continue to track rather than something the extension can override.

Two things to check

  1. System Settings → General → Login Items & Extensions → Quick Look. Confirm Syntax Look is on. Only one Quick Look extension can claim a file type, so if a media app registered .ts first, this is where the order is changed.
  2. Reset the Quick Look cache after changing anything there:
qlmanage -r && qlmanage -r cache

For everything else on a Quick Look preview that is not showing up, see the extension-not-working page.