Fix NoMethodError in FilesController when Accept header is missing (#5447)

[dantreiman]

fixes #5447

Bug

FilesController#fs starts with:

request.format = 'json' if request.headers['HTTP_ACCEPT'].split(',').include?('application/json')

If the incoming request has no Accept header, request.headers['HTTP_ACCEPT'] is nil, nil.split raises NoMethodError, the global rescue_action catches it, and the user is redirected to $HOME instead of being served the file.

How it was hit

Building an Interactive App whose view.html.erb links to a session-specific file with <a download href="/pun/sys/dashboard/files/api/v1/fs<%= abs_path %>">. Chrome 147 on macOS omits Accept on download clicks; error.log filled with undefined method 'split' for nil:NilClass. Full context in #5447.

Fix

Extract the Accept-header check into a json_request? helper that safe-navigates the split and include?. When the header is absent we simply leave request.format alone and Rails's normal content negotiation picks html:

def fs
  request.format = 'json' if json_request?
  ...
end

private

def json_request?
  request.headers['HTTP_ACCEPT']&.split(',')&.include?('application/json')
end

The extraction also slightly reduces fs's complexity scores (per rubocop: AbcSize 97.59 → 93.34, Cyclomatic 19 → 17, Perceived 25 → 23), though the method is still over the project's metric limits — preexisting debt tracked by #1187.

Tests

New controller tests in files_controller_test.rb covering the three cases:

• Accept header absent → json_request? returns nil (regression test for this issue).
• Accept contains application/json → returns truthy.
• Accept lacks application/json → returns falsy.

Integration-style get :fs tests would need extensive stubbing for an unrelated surface area; the direct helper test exercises the exact logic that was broken.

Lint

rubocop apps/dashboard/app/controllers/files_controller.rb — no new offenses introduced; 30 preexisting offenses unchanged, four of which (on fs) actually improved slightly.

Changelog

### Fixed entry added under [Unreleased].

[johrstrom]

LGTM thanks!

[Bubballoo3]

Tested and works well!