Group Avatar API: ensure to send the remote filename

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64650

lib/api/group_avatar.rb

@@ -14,6 +14,20 @@ module API
         detail 'This feature was introduced in GitLab 14.0'
       end
       get ':id/avatar' do
+        avatar = user_group.avatar
+
+        not_found!('Avatar') if avatar.blank?
+
+        filename = File.basename(avatar.file.file)
+
+        header(
+          'Content-Disposition',
+          ActionDispatch::Http::ContentDisposition.format(
+            disposition: 'attachment',
+            filename: filename
+          )
+        )
+
         present_carrierwave_file!(user_group.avatar)
       end
     end

spec/requests/api/group_avatar_spec.rb

@@ -15,6 +15,8 @@ RSpec.describe API::GroupAvatar do
         get api(avatar_path(group))
 
         expect(response).to have_gitlab_http_status(:ok)
+        expect(response.headers['Content-Disposition'])
+          .to eq(%(attachment; filename="dk.png"; filename*=UTF-8''dk.png))
       end
 
       context 'when the group does not have avatar' do
@@ -24,6 +26,8 @@ RSpec.describe API::GroupAvatar do
           get api(avatar_path(group))
 
           expect(response).to have_gitlab_http_status(:not_found)
+          expect(response.body)
+            .to eq(%({"message":"404 Avatar Not Found"}))
         end
       end