diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 3174261a9..c7ed1bf97 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -352,6 +352,7 @@ module ApplicationHelper
         :srcset => "#{thumbnail_path} 2x",
         :style => "max-width: #{thumbnail_size}px; max-height: #{thumbnail_size}px;",
         :title => attachment.filename,
+        :alt => attachment.filename,
         :loading => "lazy"
       ),
       attachment_path(
diff --git a/test/helpers/application_helper_test.rb b/test/helpers/application_helper_test.rb
index 31c87daea..dc52bf758 100644
--- a/test/helpers/application_helper_test.rb
+++ b/test/helpers/application_helper_test.rb
@@ -1921,8 +1921,8 @@ class ApplicationHelperTest < Redmine::HelperTest
     a = Attachment.find(3)
     assert_select_in(
       thumbnail_tag(a),
-      'a[href=?] img[title=?][src=?][loading="lazy"]',
-      "/attachments/3", "logo.gif", "/attachments/thumbnail/3/200")
+      'a[href=?] img[title=?][alt=?][src=?][loading="lazy"]',
+      "/attachments/3", "logo.gif", "logo.gif", "/attachments/thumbnail/3/200")
   end
 
   def test_link_to_project