expand_sources

# File actionpack/lib/action_view/helpers/asset_tag_helpers/asset_include_tag.rb, line 75
          def expand_sources(sources, recursive)
            if sources.first == :all
              collect_asset_files(custom_dir, ('**' if recursive), "*.#{extension}")
            else
              sources.inject([]) do |list, source|
                determined_source = determine_source(source, expansions)
                update_source_list(list, determined_source)
              end
            end
          end