If you work on a Mac, you know the drag-and-drop frustration: open Finder, drag a file to an app, get "Unsupported file type" — repeat with ten more files. The QuickLook API, introduced in macOS Big Sur, solves this silently. A QuickLook extension previews files in the Finder before you even open them. No app launching. No "what was that format again?"
Here are extensions I find essential for development workflows:
Developers work with PDFs, ZIP files, PKGs, DMGs, XAR archives, and ML model files (.mlmodel, .onnx, .tflite). macOS shows a generic icon for most of these. You click, the app opens, the app says "I can't open that," you close it.
A QuickLook extension previews these in the Finder. You can spot-check a configuration file in a PKG, inspect an archive's contents, or verify an ML model's metadata — without opening anything.
QuickLook extensions register file types they understand. When you press the Space key on a file in Finder, macOS asks all registered extensions if they can handle it. If one says yes, it renders a preview.
The API surface is small:
providePreview(for:withHandler:) — main entry pointicon(for:withHandler:) — custom icon (optional)fileWrapper(for:withHandler:completionHandler:) — optional file export
No UI. No windows. A single render method and a context object telling you the file URL and desired size.
OmniPreview is a free QuickLook extension with a Pro upgrade. It supports:
.mlmodel), ONNX, TensorFlow SavedModel directories
The free tier covers the essentials. Pro adds ML model inspection, archive navigation, and custom file type rules.
Let's say you download a package of configuration files. Before spending 30 minutes setting up your environment, you can preview 20 files in the Finder and spot the missing pieces. Or a ML practitioner downloads a model from Hugging Face — check its architecture and metadata instantly without a GPU.
The time saved compounds across hundreds of files per week.
QuickLook extensions live in ~/Library/QuickLook/. Installation is usually a single copy command or a drag-and-drop into that folder. No sandbox permission dialogs. No background processes.
For developers who touch dozens of file types daily, QuickLook extensions are the invisible tool that makes the Mac feel faster. Disclosure: I built OmniPreview. It's free on the Mac App Store and also available as a Gumroad version (€5+, pay-what-you-want) with source code included. The extension itself is open source.