Fix Attachment#save for binary IO

lib/ruby_llm/attachment.rb

@@ -62,9 +62,8 @@ def encoded
     def save(path)
       return unless io_like?
 
-      File.open(path, 'w') do |f|
-        f.puts(@source.read)
-      end
+      @source.rewind if @source.respond_to?(:rewind)
+      File.binwrite(path, @source.read)
     end
 
     def for_llm

spec/ruby_llm/attachment_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 require 'open3'
 require 'rbconfig'
+require 'stringio'
+require 'tempfile'
 
 RSpec.describe RubyLLM::Attachment do
   it 'supports path attachments from the public API' do
@@ -22,4 +24,17 @@
     expect(status.success?).to be(true), stderr
     expect(stdout.strip).to eq('ruby.txt,text/plain')
   end
+
+  it 'saves io attachments without altering binary content' do
+    binary_content = "\x00\xFFbinary\npayload".b
+    io = StringIO.new(binary_content)
+    io.read(2)
+    attachment = described_class.new(io, filename: 'payload.bin')
+
+    Tempfile.create('attachment') do |file|
+      attachment.save(file.path)
+
+      expect(File.binread(file.path)).to eq(binary_content)
+    end
+  end
 end