diff --git a/ts/jobs/AttachmentLocalBackupManager.preload.ts b/ts/jobs/AttachmentLocalBackupManager.preload.ts
index 3dfa1f9059..194680e72a 100644
--- a/ts/jobs/AttachmentLocalBackupManager.preload.ts
+++ b/ts/jobs/AttachmentLocalBackupManager.preload.ts
@@ -115,14 +115,15 @@ export async function runAttachmentBackupJob(
   }
 }
 
-function getExtension(
+/** @testexport */
+export function getExtension(
   contentType: string | undefined,
   fileName: string | undefined
 ): string | undefined {
   if (fileName) {
     const extension = extname(fileName).replace(/^./, '');
 
-    if (extension) {
+    if (isValidExtension(extension)) {
       return extension;
     }
   }
@@ -132,31 +133,41 @@ function getExtension(
   }
 
   if (contentType.startsWith('application/x-')) {
-    return contentType.replace('application/x-', '');
+    return normalizeExtension(contentType.replace('application/x-', ''));
   }
 
   if (contentType.startsWith('application/')) {
-    return contentType.replace('application/', '');
+    return normalizeExtension(contentType.replace('application/', ''));
   }
 
   if (contentType.startsWith('audio/')) {
-    return contentType.replace('audio/', '');
+    return normalizeExtension(contentType.replace('audio/', ''));
   }
 
   if (contentType.startsWith('image/')) {
-    return contentType.replace('image/', '');
+    return normalizeExtension(contentType.replace('image/', ''));
   }
 
   if (contentType === 'text/x-signal-plain') {
     return 'txt';
   }
   if (contentType.startsWith('text/x-')) {
-    return contentType.replace('text/x-', '');
+    return normalizeExtension(contentType.replace('text/x-', ''));
   }
 
   if (contentType.startsWith('video/')) {
-    return contentType.replace('video/', '');
+    return normalizeExtension(contentType.replace('video/', ''));
   }
 
   return undefined;
 }
+
+const VALID_EXTENSION_REGEXP = /^[A-Za-z\d](?:[\w+.-]{0,30}[A-Za-z\d])?$/;
+
+function isValidExtension(extension: string): boolean {
+  return VALID_EXTENSION_REGEXP.test(extension);
+}
+
+function normalizeExtension(extension: string): string | undefined {
+  return isValidExtension(extension) ? extension : undefined;
+}
diff --git a/ts/test-electron/services/AttachmentLocalBackupManager_test.preload.ts b/ts/test-electron/services/AttachmentLocalBackupManager_test.preload.ts
new file mode 100644
index 0000000000..41c531ae12
--- /dev/null
+++ b/ts/test-electron/services/AttachmentLocalBackupManager_test.preload.ts
@@ -0,0 +1,43 @@
+// Copyright 2026 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+import { getExtension } from '../../jobs/AttachmentLocalBackupManager.preload.ts';
+import { IMAGE_JPEG } from '../../types/MIME.std.ts';
+
+describe('AttachmentLocalBackupManager', () => {
+  describe('getExtension', () => {
+    it('prefers valid filename extensions', () => {
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.png'), 'png');
+      assert.strictEqual(getExtension(undefined, 'archive.tar.gz'), 'gz');
+      assert.strictEqual(getExtension(undefined, 'source.c'), 'c');
+      assert.strictEqual(getExtension(undefined, 'image.svg+xml'), 'svg+xml');
+    });
+
+    it('falls back to content type when the filename extension is invalid', () => {
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.*'), 'jpeg');
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment._png'), 'jpeg');
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.png_'), 'jpeg');
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.+png'), 'jpeg');
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.png+'), 'jpeg');
+      assert.strictEqual(getExtension(IMAGE_JPEG, 'attachment.png-'), 'jpeg');
+      assert.strictEqual(
+        getExtension('application/vnd.ms-excel', 'attachment.'),
+        'vnd.ms-excel'
+      );
+    });
+
+    it('ignores invalid content type extensions', () => {
+      assert.strictEqual(
+        getExtension('application/*', 'attachment.*'),
+        undefined
+      );
+      assert.strictEqual(getExtension('image/jpeg.', undefined), undefined);
+    });
+
+    it('uses txt for signal plain text attachments', () => {
+      assert.strictEqual(getExtension('text/x-signal-plain', undefined), 'txt');
+    });
+  });
+});