module ActionDispatch::TestProcess::FixtureFile

# File actionpack/lib/action_dispatch/testing/test_process.rb, line 17
def fixture_file_upload(path, mime_type = nil, binary = false)
  if self.class.respond_to?(:fixture_path) && self.class.fixture_path &&
      !File.exist?(path)
    path = File.join(self.class.fixture_path, path)
  end
  Rack::Test::UploadedFile.new(path, mime_type, binary)
end

Shortcut for Rack::Test::UploadedFile.new(File.join(ActionDispatch::IntegrationTest.fixture_path, path), type):

post :change_avatar, params: { avatar: fixture_file_upload('files/spongebob.png', 'image/png') }

To upload binary files on Windows, pass :binary as the last parameter. This will not affect other platforms:

post :change_avatar, params: { avatar: fixture_file_upload('files/spongebob.png', 'image/png', :binary) }