Gander turns the Quick Look preview in Finder into a real viewer for the files developers open every day: Markdown with Mermaid diagrams, JSON, SQLite, .env files and 30+ more types. No editor, no switching apps.
Every preview in the video is a real render from Gander’s Quick Look extension.
Gander gives each kind of file a preview built for it. Previews follow your Mac’s light or dark appearance, and every image on this page is a real render from the app.
README files open rendered, with the Raw source one click away. Everything in GitHub-flavoured Markdown works, plus the extras that usually need a browser.
$…$ and $$…$$, and footnotes:emoji: shortcodes, and mermaid code blocks drawn as diagrams
.mmd and .mermaid files open as diagrams, with the source on the next tab. Gander draws flowcharts, sequence, class, state and ER diagrams, XY charts, pie charts, mindmaps, timelines, Gantt charts, user journeys, git graphs, quadrant charts, C4 and requirement diagrams.
JSON opens instantly, even large files, with Tree and Raw views. JSON Lines and NDJSON logs show one record per line. Click a line number to expand that record as indented JSON, in the file’s own key order, and click again to fold it.
.sqlite, .sqlite3 and .db files open read-only, so the database is never modified and no journal files appear beside it. Structure lists every table’s columns, types, keys and indexes. Content shows the rows, table by table, up to the first 200 of each.
Environment files open with every value masked. Keys stay visible, and likely secrets such as passwords, tokens, API keys and private keys are marked SECRET. Values appear only when you choose Show values, and copying from the hidden view never copies a real value.
YAML, TOML, .cfg and .config files open as typed, colour-coded structures, and package.json gets an overview of its name, version, scripts and dependencies. Swift, Python, JavaScript and shell scripts are syntax-highlighted, and the line numbers stay out of anything you copy. ZIP, XIP, tar and .tar.gz archives list their contents without extracting, GZIP files show their text, and MIDI files play.
In icon, gallery and column view, Finder shows a page of the file itself: rendered Markdown, coloured JSON, YAML and code, Mermaid sources and archive contents. Thumbnails of .env files show keys only, never values.
README.md
forecast.json
architecture.mmd
docker-compose.yml
nimbus-2.4.0.zip
forecast_service.py
package.json
Cargo.tomlGander reads the file you’re previewing and nothing else. It has no account, no analytics and no networking code, and its extensions run in Apple’s App Sandbox without network access. The App Store privacy label is Data Not Collected.
Install Gander from the Mac App Store.
Open Gander once. Its window shows whether Quick Look is using it, and lets you choose which file types and Markdown extras it handles.
In Finder, select a file and press Space.
Still seeing the standard preview? Turn on Gander in System Settings › General › Login Items & Extensions › Quick Look. The Gander window has a button that opens that page.
Select any of these in Finder and press Space. The last column says whether Finder also shows a Gander thumbnail.
| Kind | Extensions | What you see | Finder thumbnail |
|---|---|---|---|
| Markdown | .md .markdown .mdown .markdn |
Rendered GitHub-flavoured Markdown with Mermaid, math, footnotes, alerts and emoji, plus the Raw source | Yes |
| Mermaid | .mmd .mermaid |
The diagram, plus the source | Yes |
| JSON | .json |
Tree and Raw views, fast even for large files | Yes |
| JSON Lines | .jsonl .ndjson |
One record per line; click a line number to expand it | Yes |
| package.json | package.json |
Name, version, scripts and dependencies | Yes |
| YAML | .yaml .yml |
A typed, colour-coded structure | Yes |
| TOML | .toml |
A typed, colour-coded structure | Yes |
| Config files | .cfg .config |
A typed, colour-coded structure | Yes |
| SQLite | .sqlite .sqlite3 .db |
Read-only Structure and Content tabs, up to 200 rows per table | No |
| Environment files | .env .env.* *.env |
Keys with values hidden until you choose Show values | Yes, keys only |
| Source code | .swift .py .js .mjs .cjs .sh .bash .zsh |
Swift, Python, JavaScript and shell with syntax highlighting and line numbers | Yes |
| Plain text | .txt .text .ini, dotfiles, files with no extension |
The text, read safely; binary files are left to macOS | Yes |
| Archives | .zip .xip .tar .tgz .tar.gz |
The list of files inside, without extracting | Yes, except XIP |
| GZIP | .gz .gzip |
The decompressed text, up to 32 KB | No |
| Folders | any folder | The list of items inside | No |
| MIDI | .mid .midi .smf .kar |
Play, pause and scrub | No |
Gander is a Quick Look extension for macOS, made by 65FN LLC. When you select a file in Finder and press Space, Gander draws a rich preview for developer files such as Markdown, Mermaid diagrams, JSON, SQLite databases and .env files. It also draws Finder thumbnails for most of them.
Any Mac with macOS 15 or later.
No. Gander has no networking code and collects no data. Previews are drawn on your Mac, and the App Store privacy label is Data Not Collected.
No. Gander opens databases read-only and immutable, so it never writes to the file or creates -wal, -shm or journal files beside it. Rows that exist only in an uncommitted -wal file aren’t shown, and the preview says so.
Only when you ask. Every value is hidden when the preview opens; keys stay visible, and likely secrets are marked SECRET. Choose Show values to reveal them. Finder thumbnails of .env files show keys only.
Yes. A fenced code block marked mermaid is drawn as a diagram inside the rendered page, and .mmd or .mermaid files open as diagrams on their own.
Open Gander once, then go to System Settings › General › Login Items & Extensions › Quick Look and make sure Gander is turned on. If another app previews the same file type, turn that one off. Then select the file in Finder and press Space again.
macOS doesn’t let third-party thumbnail extensions handle SQLite files, so Finder keeps its own icon. Pressing Space still opens the full Gander preview.
Yes. The Gander window has a switch for each file category and for each Markdown extra: math, footnotes, GitHub alerts and emoji. Switch a category off and Gander stops handling that type.
Quick Look previews can’t load images, so Gander shows each image as a picture icon and its alt text.