Attaching ActiveStorage files in factorybot

This solution works with rails 6:

factory :post do
  # ...
  after(:build) do |post|
    post.image.attach(
      io: File.open(Rails.root.join('test', 'fixture_files', 'test.jpg')),
      filename: 'test.jpg',
      content_type: 'image/jpeg'
    )
  end
end

Change your code to the following it will work.

include ActionDispatch::TestProcess
factory :activity_fit_file, class: 'Activity' do
    association :user, factory: :user
    activity_type {:cycling}
    original_activity_log_file { fixture_file_upload("#{Rails.root}/spec/files/example_fit_file.fit") }
end

Try with Rack::Test::UploadedFile directly:

file { Rack::Test::UploadedFile.new('path', 'image/png') }