klass.rb

class Klass
  def self.foo
    File.open "filename", "w" do |file|
      file.write("text")
    end
  end
end

klass_spec.rb

describe "foo" do

  it "should create 'filename' and put 'text' in it" do
    file = mock('file')
    File.should_receive(:open).with("filename", "w").and_yield(file)
    file.should_receive(:write).with("text")
    foo
  end

end