1 files changed, 13 insertions, 0 deletions
diff --git a/src/data/composite/wiki-properties/fileExtension.js b/src/data/composite/wiki-properties/fileExtension.js
new file mode 100644
index 0000000..c926fa8
--- /dev/null
+++ b/src/data/composite/wiki-properties/fileExtension.js
@@ -0,0 +1,13 @@
+// A file extension! Or the default, if provided when calling this.
+
+import {isFileExtension} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function(defaultFileExtension = null) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isFileExtension},
+    expose: {transform: (value) => value ?? defaultFileExtension},
+  };
+}