Fix uploading of file with no extension/MIME type using rack_test driver

lib/capybara/rack_test/form.rb

@@ -42,7 +42,8 @@ def params(button)
               if (value = field['value']).to_s.empty?
                 NilUploadedFile.new
               else
-                Rack::Test::UploadedFile.new(value, MiniMime.lookup_by_filename(value).content_type)
+                mime_info = MiniMime.lookup_by_filename(value)
+                Rack::Test::UploadedFile.new(value, (mime_info && mime_info.content_type).to_s)
               end
             merge_param!(params, field['name'].to_s, file)
           else

lib/capybara/spec/fixtures/no_extension

@@ -0,0 +1 @@
+ThisFileHAsNoExtension

lib/capybara/spec/session/attach_file_spec.rb

@@ -4,6 +4,7 @@
     @test_file_path = File.expand_path('../fixtures/test_file.txt', File.dirname(__FILE__))
     @another_test_file_path = File.expand_path('../fixtures/another_test_file.txt', File.dirname(__FILE__))
     @test_jpg_file_path = File.expand_path('../fixtures/capybara.jpg', File.dirname(__FILE__))
+    @no_extension_file_path = File.expand_path('../fixtures/no_extension', File.dirname(__FILE__))
     @session.visit('/form')
   end
 
@@ -57,6 +58,12 @@
       expect(@session).to have_content('image/jpeg')
     end
 
+    it "should not break when uploading a file without extension" do
+      @session.attach_file "Single Document", @no_extension_file_path
+      @session.click_button 'Upload Single'
+      expect(@session).to have_content(File.read(@no_extension_file_path))
+    end
+
     it "should not break when using HTML5 multiple file input" do
       @session.attach_file "Multiple Documents", @test_file_path
       @session.click_button('Upload Multiple')