Only expand the path if not already absolute

lib/capybara/selenium/node.rb

@@ -300,7 +300,10 @@ private
 
   def set_file(value) # rubocop:disable Naming/AccessorMethodName
     path_names = value.to_s.empty? ? [] : value
-    native.send_keys(Array(path_names).map(&File.method(:expand_path)).join("\n"))
+    file_names = Array(path_names).map do |pn|
+      Pathname.new(pn).absolute? ? pn : File.expand_path(pn)
+    end.join("\n")
+    native.send_keys(file_names)
   end
 
   def set_content_editable(value) # rubocop:disable Naming/AccessorMethodName

spec/shared_selenium_session.rb

@@ -323,6 +323,8 @@ RSpec.shared_examples 'Capybara::Session' do |session, mode|
       end
 
       it 'can attach a relative file' do
+        pending 'Geckdoriver on windows requires alternate file separator which path expansion replaces' if Gem.win_platform? && firefox?(session)
+
         session.visit('/form')
         session.attach_file('Single Document', 'spec/fixtures/capybara.csv')
         session.click_button('Upload Single')