diff --git a/lib/api/helpers/attachment_renderer.rb b/lib/api/helpers/attachment_renderer.rb
index 440b0935aa3..25ae274267b 100644
--- a/lib/api/helpers/attachment_renderer.rb
+++ b/lib/api/helpers/attachment_renderer.rb
@@ -94,6 +94,8 @@ module API
 
       def attachment_content_type(attachment)
         if attachment.is_text?
+          # Even if the text mime type might differ, always output plain text
+          # so this doesn't get interpreted as e.g., a script or html file
           "text/plain"
         elsif attachment.inlineable?
           attachment.content_type
diff --git a/spec/features/security/plain_text_content_type.rb b/spec/features/security/plain_text_content_type.rb
index 1179304f25b..465aa49cdbb 100644
--- a/spec/features/security/plain_text_content_type.rb
+++ b/spec/features/security/plain_text_content_type.rb
@@ -28,47 +28,47 @@
 
 require "spec_helper"
 
-RSpec.describe "Session TTL",
-               with_settings: { session_ttl_enabled?: true, session_ttl: "10" } do
+RSpec.describe "Plain text content type XSS prevention", :js, :with_cuprite do
   shared_let(:admin) { create(:admin) }
-  let(:admin_password) { "adminADMIN!" }
+  shared_let(:work_package) { create(:work_package) }
 
-  let!(:work_package) { create(:work_package) }
+  let(:wp_page) { Pages::FullWorkPackage.new(work_package) }
+  let(:attachments) { Components::Attachments.new }
+  let(:attachments_list) { Components::AttachmentsList.new }
+  let(:image_fixture) { UploadedFile.load_from("spec/fixtures/files/test.js") }
 
   before do
-    login_with(admin.login, admin_password)
+    login_as admin
   end
 
-  def expire!
-    page.set_rack_session(updated_at: Time.now - 1.hour)
-  end
+  it "allows accessing text/javascript files as inlinable plain text" do
+    wp_page.visit_tab!(:files)
+    attachments_list.wait_until_visible
 
-  describe "outdated TTL on Rails request" do
-    it "expires on the next Rails request" do
-      visit "/my/account"
-      expect(page).to have_css(".form--field-container", text: admin.login)
+    ##
+    # Attach file manually
+    attachments_list.expect_empty
+    attachments.attach_file_on_input(image_fixture.path)
+    attachments_list.expect_attached("test.js")
 
-      # Expire the session
-      expire!
+    expect(work_package.attachments.count).to eq 1
+    attachment = work_package.attachments.first
 
-      visit "/"
-      expect(page).to have_css(".action-login")
-    end
-  end
+    expect(attachment.content_type).to eq "text/x-javascript"
+    expect(attachment).to be_inlineable
 
-  describe "outdated TTL on API request" do
-    it "expires on the next APIv3 request" do
-      page.driver.header("X-Requested-With", "XMLHttpRequest")
-      visit "/api/v3/work_packages/#{work_package.id}"
+    visit home_path
 
-      body = JSON.parse(page.body)
-      expect(body["id"]).to eq(work_package.id)
-
-      # Expire the session
-      expire!
-      visit "/api/v3/work_packages/#{work_package.id}"
-
-      expect(page.body).to eq("unauthorized")
-    end
+    # Assume we have a HTML sanitation issue
+    expect do
+      accept_alert(wait: 1) do
+        page.execute_script <<-JS
+      const element = document.createElement('script')
+      element.id = "testscript"
+      element.src = '/api/v3/attachments/#{attachment.id}/content'
+      document.body.appendChild(element);
+        JS
+      end
+    end.to raise_error(Capybara::ModalNotFound)
   end
 end
diff --git a/spec/fixtures/files/test.js b/spec/fixtures/files/test.js
new file mode 100644
index 00000000000..3254c929467
--- /dev/null
+++ b/spec/fixtures/files/test.js
@@ -0,0 +1 @@
+alert("hello");
diff --git a/spec/requests/api/v3/attachments/attachment_resource_shared_examples.rb b/spec/requests/api/v3/attachments/attachment_resource_shared_examples.rb
index 10d9a529335..cc32297a87d 100644
--- a/spec/requests/api/v3/attachments/attachment_resource_shared_examples.rb
+++ b/spec/requests/api/v3/attachments/attachment_resource_shared_examples.rb
@@ -219,6 +219,8 @@ RSpec.shared_examples "an APIv3 attachment resource", content_type: :json, type:
 
   let(:missing_permissions_user) { user_with_permissions }
 
+  let(:expected_content_type) { "application/octet-stream" }
+
   before do
     allow(User).to receive(:current).and_return current_user
   end
@@ -452,7 +454,7 @@ RSpec.shared_examples "an APIv3 attachment resource", content_type: :json, type:
             .to eql content_disposition
 
           expect(subject.headers["Content-Type"])
-            .to eql mock_file.content_type
+            .to eql expected_content_type
 
           max_age = OpenProject::Configuration.fog_download_url_expires_in.to_i - 10
 
@@ -473,11 +475,20 @@ RSpec.shared_examples "an APIv3 attachment resource", content_type: :json, type:
 
       context "for a local text file" do
         it_behaves_like "for a local file" do
+          let(:expected_content_type) { "text/plain" }
           let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.txt" }
           let(:content_disposition) { "inline; filename=foobar.txt" }
         end
       end
 
+      context "for a local JS file" do
+        it_behaves_like "for a local file" do
+          let(:expected_content_type) { "text/plain" }
+          let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.js", content_type: "text/x-javascript" }
+          let(:content_disposition) { "inline; filename=foobar.js" }
+        end
+      end
+
       context "for a local binary file" do
         it_behaves_like "for a local file" do
           let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.dat", content_type: "application/octet-stream" }