[Fix #1066] Fix an error for Rails/FilePath

Fixes #1066.

This PR fixes an error for `Rails/FilePath`
when string interpolated `Rails.root` is followed by a message starting with `.`.

Author: koic

changelog/fix_an_error_for_rails_file_path.md

@@ -0,0 +1 @@
+* [#1066](https://github.com/rubocop/rubocop-rails/issues/1066): Fix an error for `Rails/FilePath` when string interpolated `Rails.root` is followed by a message starting with `.`. ([@koic][])

lib/rubocop/cop/rails/file_path.rb

@@ -233,7 +233,7 @@ def extract_rails_root_join_argument_source(node, rails_root_index)
         end
 
         def extension_node?(node)
-          node&.str_type? && node.source.start_with?('.')
+          node&.str_type? && node.source.match?(/\A\.[A-Za-z]+/)
         end
       end
     end

spec/rubocop/cop/rails/file_path_spec.rb

@@ -64,6 +64,14 @@
       end
     end
 
+    context 'when string interpolated `Rails.root` is followed by a message starting with `.`' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~'RUBY')
+          "#{Rails.root}. a message"
+        RUBY
+      end
+    end
+
     context 'when using string interpolation without Rails.root' do
       it 'does not register an offense' do
         expect_no_offenses(<<~'RUBY')