Fixes issue with multipart forms being submitted as x-www-form-urlencoded when they contain an empty file field

lib/capybara/driver/rack_test_driver.rb

@@ -97,6 +97,21 @@ class Capybara::Driver::RackTest < Capybara::Driver::Base
   end
 
   class Form < Node
+    # This only needs to inherit from Rack::Test::UploadedFile because Rack::Test checks for
+    # the class specifically when determing whether to consturct the request as multipart.
+    # That check should be based solely on the form element's 'enctype' attribute value,
+    # which should probably be provided to Rack::Test in its non-GET request methods.
+    class NilUploadedFile < Rack::Test::UploadedFile
+      def initialize
+        @empty_file = Tempfile.new("nil_uploaded_file")
+        @empty_file.close
+      end
+
+      def original_filename; ""; end
+      def content_type; "application/octet-stream"; end
+      def path; @empty_file.path; end
+    end
+
     def params(button)
       params = {}
 
@@ -122,14 +137,17 @@ class Capybara::Driver::RackTest < Capybara::Driver::Base
         end
       end
       native.xpath(".//input[not(@disabled) and @type='file']").map do |input|
-        unless input['value'].to_s.empty?
-          if multipart?
-            content_type = MIME::Types.type_for(input['value'].to_s).first.to_s
-            file = Rack::Test::UploadedFile.new(input['value'].to_s, content_type)
-            merge_param!(params, input['name'].to_s, file)
-          else
-            merge_param!(params, input['name'].to_s, File.basename(input['value'].to_s))
-          end
+        if multipart?
+          file = \
+            if (value = input['value']).to_s.empty?
+              NilUploadedFile.new
+            else
+              content_type = MIME::Types.type_for(value).first.to_s
+              Rack::Test::UploadedFile.new(value, content_type)
+            end
+          merge_param!(params, input['name'].to_s, file)
+        else
+          merge_param!(params, input['name'].to_s, File.basename(input['value'].to_s))
         end
       end
       merge_param!(params, button[:name], button[:value] || "") if button[:name]

lib/capybara/spec/test_app.rb

@@ -77,6 +77,14 @@ class TestApp < Sinatra::Base
     '<pre id="results">' + params[:form].to_yaml + '</pre>'
   end
 
+  post '/upload_empty' do
+    if params[:form][:file].nil?
+      'Successfully ignored empty file field.'
+    else
+      'Something went wrong.'
+    end
+  end
+
   post '/upload' do
     begin
       buffer = []

lib/capybara/spec/views/form.erb

@@ -250,6 +250,17 @@
   <p>
 </form>
 
+<form action="/upload_empty" method="post" enctype="multipart/form-data">
+  <p>
+    <label for="form_file_name">File Name</label>
+    <input type="file" name="form[file]" id="form_file"/>
+  </p>
+
+  <p>
+    <input type="submit" value="Upload Empty"/>
+  <p>
+</form>
+
 <form action="/upload" method="post" enctype="multipart/form-data">
   <p>
     <label for="form_file_name">File Name</label>

spec/session/rack_test_session_spec.rb

@@ -26,6 +26,16 @@ describe Capybara::Session do
       end
     end
 
+    describe "#attach_file" do
+      context "with multipart form" do
+        it "should submit an empty form-data section if no file is submitted" do
+          @session.visit("/form")
+          @session.click_button("Upload Empty")
+          @session.body.should include('Successfully ignored empty file field.')
+        end
+      end
+    end
+
     it_should_behave_like "session"
     it_should_behave_like "session without javascript support"
     it_should_behave_like "session with headers support"