Support file_fixture in Factory definitions

Related to [factory_bot#1282][]

[rails/rails#45606][] has been merged and is likely to be released as
part of Rails 7.1.

With that addition, the path toward resolving [factory_bot#1282][]
becomes more clear. If factories can pass along [Pathname][] instances
to attachment attributes, Active Support will handle the rest.

Instances of `ActiveSupport::TestCase` provide a [file_fixture][] helper
to construct a `Pathname` instance based on the path defined by
`ActiveSupport::TestCase.file_fixture_path` (relative to the Rails root
directory).

[factory_bot#1282]: thoughtbot/factory_bot#1282 (comment)
[rails/rails#45606]: rails/rails#45606
[Pathname]: https://docs.ruby-lang.org/en/master/Pathname.html
[file_fixture]: https://api.rubyonrails.org/classes/ActiveSupport/Testing/FileFixtures.html#method-i-file_fixture

lib/factory_bot_rails/railtie.rb

@@ -5,6 +5,7 @@
 require "factory_bot_rails/reloader"
 require "factory_bot_rails/factory_validator"
 require "rails"
+require "active_support/testing/file_fixtures"
 
 module FactoryBotRails
   class Railtie < Rails::Railtie
@@ -31,6 +32,22 @@ class Railtie < Rails::Railtie
       end
     end
 
+    config.after_initialize do
+      FactoryBot::SyntaxRunner.include ActiveSupport::Testing::FileFixtures
+
+      ActiveSupport.on_load :active_support_test_case do
+        FactoryBot::SyntaxRunner.file_fixture_path = file_fixture_path
+      end
+
+      if defined?(RSpec)
+        RSpec.configure do |config|
+          if config.respond_to?(:file_fixture_path)
+            FactoryBot::SyntaxRunner.file_fixture_path = config.file_fixture_path
+          end
+        end
+      end
+    end
+
     config.after_initialize do |app|
       FactoryBot.find_definitions
       Reloader.new(app).run

spec/factory_bot_rails/factory_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+describe "factory extensions" do
+  describe "#file_fixture" do
+    it "delegates to the test harness" do
+      define_model "Upload", filename: :string
+
+      FactoryBot.define do
+        factory :upload do
+          filename { file_fixture("file.txt") }
+        end
+      end
+
+      upload = FactoryBot.build(:upload)
+
+      expect(upload.filename).to eq(file_fixture("file.txt").to_s)
+    end
+  end
+end