"File not found" error raised on file that exists

I’m running a CircleCI build; Rails, Ruby 2.0, RSpec & Capybara. For some reason, specs related to files are failing, claiming that the files that do exist in the distro actually don’t exist. For example, this spec which passes in the master branch of our codebase:

it "should respond to fileupload" do
  stub_request(:post, %r{.*/uploads.xml}).
  to_return(status: 201, body: adgear_file("create_file_with_uploads.xml"))
  @file = fixture_file_upload("spec/fixtures/adgear_data/demo.gif", "image/gif")

  post :file_upload, fileupload: @file, campaign_id: @campaign.id, format: "json"
  response.status.should == 200

Fails with the following error in my feature branch:

The file is set to be readable by the build user, but there is something that is causing the file read to fail.

Any thoughts or ideas appreciated.