diff --git a/src/archiveExtensions.js b/src/archiveExtensions.js
new file mode 100644
index 0000000..7641276
--- /dev/null
+++ b/src/archiveExtensions.js
@@ -0,0 +1,14 @@
+import path from 'node:path';
+
+const DOUBLE_EXTENSIONS = ['tar.gz', 'tar.bz2', 'tar.7z'];
+
+export function extractExtension(filename) {
+  const lower = filename.toLowerCase();
+  const match = DOUBLE_EXTENSIONS.find((ext) => lower.endsWith(`.${ext}`));
+  return match ?? path.extname(filename).slice(1).toLowerCase();
+}
+
+export function stripExtension(filename) {
+  const ext = extractExtension(filename);
+  return filename.slice(0, filename.length - ext.length - 1);
+}
diff --git a/test/archiveExtensions.test.js b/test/archiveExtensions.test.js
new file mode 100644
index 0000000..7ef6c33
--- /dev/null
+++ b/test/archiveExtensions.test.js
@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { extractExtension, stripExtension } from '../src/archiveExtensions.js';
+
+describe('extractExtension', () => {
+  it('returns the simple extension for a normal filename', () => {
+    expect(extractExtension('photo.PNG')).toBe('png');
+  });
+
+  it('recognizes tar.gz as a single compound extension', () => {
+    expect(extractExtension('backup.TAR.GZ')).toBe('tar.gz');
+  });
+
+  it('recognizes tar.bz2 as a single compound extension', () => {
+    expect(extractExtension('backup.tar.bz2')).toBe('tar.bz2');
+  });
+
+  it('recognizes tar.7z as a single compound extension', () => {
+    expect(extractExtension('backup.tar.7z')).toBe('tar.7z');
+  });
+
+  it('does not treat an unrelated double extension as compound', () => {
+    expect(extractExtension('archive.zip.bak')).toBe('bak');
+  });
+});
+
+describe('stripExtension', () => {
+  it('strips a simple extension', () => {
+    expect(stripExtension('photo.png')).toBe('photo');
+  });
+
+  it('strips a compound tar.gz extension, keeping the rest of the name intact', () => {
+    expect(stripExtension('my.backup.tar.gz')).toBe('my.backup');
+  });
+});