Get FileUploader into test harness using factory

This attempts to get CarrierWave's uploader - `FileUploader` into test
harness using a factory. that makes it easier to build an instance of
it.

Along with !3435 it may be easier to use uploaders in tests

app/uploaders/file_uploader.rb

@@ -7,9 +7,9 @@ class FileUploader < CarrierWave::Uploader::Base
 
   attr_accessor :project, :secret
 
-  def initialize(project, secret = self.class.generate_secret)
+  def initialize(project, secret = nil)
     @project = project
-    @secret = secret
+    @secret = secret || self.class.generate_secret
   end
 
   def base_dir

spec/factories/file_uploader.rb

+FactoryGirl.define do
+  factory :file_uploader, class: FileUploader do
+    project
+    secret nil
+
+    transient do
+      path { File.join(Rails.root, 'spec/fixtures/rails_sample.jpg') }
+      file { Rack::Test::UploadedFile.new(path) }
+    end
+
+    after(:build) do |uploader, evaluator|
+      uploader.store!(evaluator.file)
+    end
+
+    initialize_with do
+      new(project, secret)
+    end
+  end
+end

spec/lib/gitlab/gfm/uploads_rewriter_spec.rb

@@ -7,10 +7,7 @@ describe Gitlab::Gfm::UploadsRewriter do
   let(:rewriter) { described_class.new(text, old_project, user) }
 
   context 'text contains links to uploads' do
-    let(:path) { Rails.root + 'spec/fixtures/rails_sample.jpg' }
-    let(:file) { fixture_file_upload(path, 'image/jpg') }
-    let(:uploader) { FileUploader.new(old_project) }
-    let!(:store) { uploader.store!(file) } # TODO, see #xxx (carrierwave issue)
+    let(:uploader) { build(:file_uploader, project: old_project) }
     let(:markdown) { uploader.to_h[:markdown] }
     let(:text) { "Text and #{markdown}"}