user_api_request_spec.rb

require 'spec_helper'

describe Users do

  describe 'Update user' do
    let(:user) { FactoryGirl.create(:user) }
    let(:image) {
      Rack::Test::UploadedFile.new('spec/fixtures/images/treme.jpg', 'image/jpeg')
    }

    before do
      post "/api/users/#{user.id}", {
          avatar: {
            :content_type => image.content_type,
            :filename => image.original_filename,
            :file_data => Base64.encode64(image.read)
          }
        }.to_json, "Content-Type" => "application/json"
    end

    it 'returns avatar URL' do
      JSON.parse(response.body)['avatar_url'].should_not be_nil
    end
  end
end

user_update_service.rb

class UserUpdateService

  def initialize(user, params)
    @user = user
    @params = params
  end

  def process
    @params[:avatar] = parse_image_data(@params[:avatar]) if @params[:avatar]
    @user.update!(@params)
  ensure
    clean_tempfile
  end

  private

  def parse_image_data(image_data)
    @tempfile = Tempfile.new('user-avatar')
    @tempfile.binmode
    @tempfile.write Base64.decode64(image_data[:file_data])
    @tempfile.rewind

    ActionDispatch::Http::UploadedFile.new(
      :tempfile => @tempfile,
      :content_type => image_data[:content_type],
      :filename => image_data[:filename]
    )
  end

  def clean_tempfile
    if @tempfile
      @tempfile.close
      @tempfile.unlink
    end
  end

end