diff --git a/static_src/shared/settings.js b/static_src/shared/settings.js
new file mode 100644
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+++ b/static_src/shared/settings.js
@@ -0,0 +1,23 @@
+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// List of content type prefixes that the user will not be warned about when
+// downloading an attachment.
+export const ALLOWED_CONTENT_TYPE_PREFIXES = [
+  'audio/', 'font/', 'image/', 'text/plain', 'video/',
+];
+
+// List of file extensions that the user will not be warned about when
+// downloading an attachment.
+export const ALLOWED_ATTACHMENT_EXTENSIONS = [
+  '.avi', '.avif', '.bmp', '.csv', '.doc', '.docx', '.email', '.eml', '.gif',
+  '.ico', '.jpeg', '.jpg', '.log', '.m4p', '.m4v', '.mkv', '.mov', '.mp2',
+  '.mp4', '.mpeg', '.mpg', '.mpv', '.odt', '.ogg', '.pdf', '.png', '.sql',
+  '.svg', '.tif', '.tiff', '.txt', '.wav', '.webm', '.wmv',
+];
+
+// The message to show the user when they attempt to download an unrecognized
+// file type.
+export const FILE_DOWNLOAD_WARNING = 'This file type is not recognized. Are' +
+  ' you sure you want to download this attachment?';