Backport UploadedFile keyword forwarding fix

Backport the fix from rack#2482 to Rack 3.2, retaining ruby2_keywords because the stable branch supports Ruby 2.4.

Author: SeanLF

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file. For info on how to format all future additions to this file please reference [Keep A Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
+### Fixed
+
+- `Rack::Multipart::UploadedFile` now delegates keyword arguments to the wrapped tempfile. Calls such as `uploaded_file.readlines(chomp: true)` raised `TypeError` on Ruby 3.0+. ([#2481](https://github.com/rack/rack/issues/2481), [@SeanLF](https://github.com/SeanLF))
+
 ## [3.2.7] - 2026-08-13
 
 ### Fixed

lib/rack/multipart/uploaded_file.rb

@@ -77,6 +77,7 @@ def respond_to_missing?(*args)
       def method_missing(method_name, *args, &block) #:nodoc:
         @tempfile.__send__(method_name, *args, &block)
       end
+      ruby2_keywords(:method_missing) if respond_to?(:ruby2_keywords, true)
     end
   end
 end

test/spec_multipart.rb

@@ -1019,6 +1019,13 @@ def initialize(*)
     Rack::Multipart::UploadedFile.new(multipart_file("file1.txt"), binary: true).must_be :binmode?
   end
 
+  it "delegates keyword arguments to the tempfile" do
+    file = Rack::Multipart::UploadedFile.new(multipart_file("file1.txt"))
+    file.readlines(chomp: true).must_equal ['contents']
+    file.rewind
+    file.gets(chomp: true).must_equal 'contents'
+  end
+
   it "builds multipart body" do
     files = Rack::Multipart::UploadedFile.new(multipart_file("file1.txt"))
     data  = Rack::Multipart.build_multipart("submit-name" => "Larry", "files" => files)