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Adding a new file extension

Two files need to change, plus a rebuild. The order matters.

Before you start — check the UTI

Run this in Terminal (replace e42 with your extension):

touch /tmp/test.e42
mdls -name kMDItemContentType -name kMDItemContentTypeTree /tmp/test.e42
kMDItemContentType result What it means Which path to follow
dyn.ah62… (dynamic) No app or system owns this extension yet Common path — Steps 1 + 2 below
A named UTI (e.g. public.yaml) System or an installed app already declared it Named UTI path — Step 2 only (see note)
A named video/audio UTI A system handler may beat your extension Check the Known Limitations section in README.md

Common path — dynamic UTI (most unknown extensions)

Step 1 — Declare the extension in the host app's Info.plist

Open QLTextView/Info.plist. Find the UTExportedTypeDeclarations array and add your extension alongside the existing entries:

<key>public.filename-extension</key>
<array>
    <string>jq</string>
    <string>jsonl</string>
    <!-- add yours here -->
    <string>e42</string>
    ...
</array>

This binds your extension to the com.qltextview.app.previewable-text UTI (which conforms to public.plain-text), so Quick Look routes the file to the extension. Without this step, routing silently fails for dynamic-UTI files.

Step 2 — Add to the in-code allowlist

Open QLTextViewExtension/TextDetection.swift and add your extension to defaultExtensions:

static let defaultExtensions: Set<String> = [
    "yaml", "yml", "toml", "json", "jsonl", "jq",
    "e42",   // ← add here
    ...
]

This documents the intent and is the last guard before rendering: even if routing works, the byte-sniff will reject binary files with your extension.

Step 3 — Rebuild and install

just build && just sign && just install

Test in Finder: select a file with your new extension and press Space.

Named UTI path

If mdls returned a named UTI (not dyn.*), the extension is already bound to a known type. Skip Step 1 (Info.plist). Instead, check whether that UTI is already declared in QLTextViewExtension/Info.plist under QLSupportedContentTypes. If it is, only Step 2 is needed. If it isn't, add the UTI string to that array as well.

Does this require a new release?

Yes. Both Info.plist (baked into the bundle at build time) and TextDetection.swift (compiled Swift) require a rebuild. The fast loop (just build && just sign && just install) is enough for local use. For distributing to other machines, run just dist.

Phase 2 note: When user-editable configuration is implemented, extensions whose UTI is already covered by the existing QLSupportedContentTypes declarations will be addable at runtime without a rebuild. Extensions that need a new UTExportedTypeDeclarations entry will always require a new build.