Just remove the prefix, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5094#note_12987385

features/steps/project/builds/artifacts.rb

@@ -72,7 +72,8 @@ class Spinach::Features::ProjectBuildsArtifacts < Spinach::FeatureSteps
 
     expect(send_data).to start_with('artifacts-entry:')
 
-    params = JSON.parse(Base64.urlsafe_decode64(send_data[/(?<=:)(.+)/]))
+    base64_params = send_data.sub(/\Aartifacts\-entry:/, '')
+    params = JSON.parse(Base64.urlsafe_decode64(base64_params))
 
     expect(params.keys).to eq(['Archive', 'Entry'])
     expect(params['Archive']).to end_with('build_artifacts.zip')