Allow JSON and IO in the same payload

lib/faraday/request/multipart.rb

@@ -52,7 +52,7 @@ def has_multipart?(obj) # rubocop:disable Naming/PredicateName
       def create_multipart(env, params)
         boundary = env.request.boundary
         parts = process_params(params) do |key, value|
-          Faraday::Parts::Part.new(boundary, key, value)
+          part(boundary, key, value)
         end
         parts << Faraday::Parts::EpiloguePart.new(boundary)
 
@@ -61,6 +61,21 @@ def create_multipart(env, params)
         body
       end
 
+      def part(boundary, key, value)
+        if json?(value)
+          header = { 'Content-Type' => 'application/json' }
+          Faraday::Parts::Part.new(boundary, key, value, header)
+        else
+          Faraday::Parts::Part.new(boundary, key, value)
+        end
+      end
+
+      def json?(value)
+        JSON.parse(value)
+      rescue StandardError
+        false
+      end
+
       # @return [String]
       def unique_boundary
         "#{DEFAULT_BOUNDARY_PREFIX}-#{SecureRandom.hex}"

spec/faraday/request/multipart_spec.rb

@@ -68,6 +68,45 @@
     end
   end
 
+  context 'when providing json and IO content in the same payload' do
+    let(:io_content) { StringIO.new('io-content') }
+    let(:json_content) do
+      {
+        b: 1,
+        c: 2
+      }.to_json
+    end
+
+    let(:payload) do
+      {
+        json_content: json_content,
+        io_content: Faraday::UploadIO.new(io_content, 'application/pdf')
+      }
+    end
+
+    it_behaves_like 'a multipart request'
+
+    it 'forms a multipart request' do
+      response = conn.post('/echo', payload)
+
+      boundary = parse_multipart_boundary(response.headers['Content-Type'])
+      result = parse_multipart(boundary, response.body)
+      expect(result[:errors]).to be_empty
+
+      part_json, body_json = result.part('json_content')
+      expect(part_json).to_not be_nil
+      expect(part_json.mime).to eq('application/json')
+      expect(part_json.filename).to be_nil
+      expect(body_json).to eq(json_content)
+
+      part_io, body_io = result.part('io_content')
+      expect(part_io).to_not be_nil
+      expect(part_io.mime).to eq('application/pdf')
+      expect(part_io.filename).to eq('local.path')
+      expect(body_io).to eq(io_content.string)
+    end
+  end
+
   context 'when multipart objects in array param' do
     let(:payload) do
       {