diff --git a/data/settings.js b/data/settings.js
index 16e75ab45..a69e219a9 100644
--- a/data/settings.js
+++ b/data/settings.js
@@ -7427,6 +7427,11 @@ See [[link,mail_cache]] for details and for the list of fields.`
 
 	mail_attachment_detection_options: {
 		values: setting_types.BOOLLIST,
+		default: `
+- mbox, imapc: &lt;empty&gt;<br />
+- others: add-flags content-type=!application/signature
+<br /><br />
+[[changed,mail_attachment_detection_options_changed]] The default was set.`,
 		text: `
 Settings to control adding \`$HasAttachment\` or \`$HasNoAttachment\`
 keywords. By default, all MIME parts with \`Content-Disposition=attachment\`
diff --git a/data/updates.js b/data/updates.js
index c4f363b36..0d44ed8c6 100644
--- a/data/updates.js
+++ b/data/updates.js
@@ -52,6 +52,7 @@ export const updates = {
 	lua_auth_init: '2.4.0',
 	lua_dns_client: '2.4.0',
 	lua_script_init: '2.4.0',
+	mail_attachment_detection_options_changed: '2.4.2',
 	mail_cache_fields_changed: '2.4.1',
 	mail_crypt_eddsa: '2.4.0',
 	mail_crypt_fs_maybe: '2.4.0',
diff --git a/docs/installation/upgrade/2.4-to-2.4.x.md b/docs/installation/upgrade/2.4-to-2.4.x.md
index 3f0c0132c..d9cdc0ffc 100644
--- a/docs/installation/upgrade/2.4-to-2.4.x.md
+++ b/docs/installation/upgrade/2.4-to-2.4.x.md
@@ -39,3 +39,4 @@ changed to `2.4.2`.
 | `service/anvil/unix_listener/anvil/mode` | `0600` | `0660` |
 | `service/anvil/unix_listener/anvil/group` | (empty = root) | `$SET:default_internal_group` |
 | [[setting,mailbox_directory_name_legacy]] | `yes` | `no` |
+| [[setting,mail_attachment_detection_options]] |  | `add-flags content-type=!application/signature` |