Add a macro for complexes multiparts

This will allow to describe multipart in a short way, like JSON payload:

Before:

    put 'Creates a blog with thumbnail' do
      consumes 'multipart/form-data'
      parameter name: :title, in: :formData, type: :string, required: true
      parameter name: :content, in: :formData, type: :string, required: true
      parameter name: :file, in: :formData, type: :file, required: true

      let(:blog) { FactoryBot.build(:blog) }
      let(:title) { blog.title }
      let(:content) { blog.content }
      let(:file) { blog.file }

      ...
    end

After:

    put 'Creates a blog with thumbnail' do
      consumes 'multipart/form-data'
      parameter name: :blog, in: :formData, schema: { '$ref' => '#/definitions/blog' }

      let(:blog) { FactoryBot.attributes_for(:blog) }

      ...
    end

Your mileage may vary but you can always choose the best option.

rswag-specs/lib/rswag/specs/request_factory.rb

@@ -185,8 +185,22 @@ module Rswag
         # Rather that serializing with the appropriate encoding (e.g. multipart/form-data),
         # Rails test infrastructure allows us to send the values directly as a hash
         # PROS: simple to implement, CONS: serialization/deserialization is bypassed in test
+        smart_payload = build_smart_form_payload(parameters, example)
+        raw_payload = build_raw_form_payload(parameters, example)
+
+        smart_payload.merge(raw_payload)
+      end
+
+      def build_smart_form_payload(parameters, example)
+        smart_tuples = parameters
+          .select { |p| p[:in] == :formData && p[:schema] }
+          .map { |p| example.send(p[:name]) }
+          .reduce({}, :merge)
+      end
+
+      def build_raw_form_payload(parameters, example)
         tuples = parameters
-          .select { |p| p[:in] == :formData }
+          .select { |p| p[:in] == :formData && !p[:schema] }
           .map { |p| [p[:name], example.send(p[:name])] }
         Hash[tuples]
       end