Added feature test

spec/features/projects/blobs/balsamiq_preview_spec.rb

+require 'spec_helper'
+
+feature 'Balsamiq preview', :feature, :js do
+  include TreeHelper
+
+  let(:user) { create(:user) }
+  let(:project) { create(:project) }
+  let(:branch) { 'add-balsamiq-file' }
+  let(:path) { 'files/images/balsamiq.bmpr' }
+
+  before do
+    project.add_master(user)
+
+    login_as user
+
+    p namespace_project_blob_path(project.namespace, project, tree_join(branch, path))
+
+    visit namespace_project_blob_path(project.namespace, project, tree_join(branch, path))
+  end
+
+  it 'should' do
+    screenshot_and_open_image
+  end
+end