Fix false positive test in cms acceptance

This test was passing in Jenkins, but failing in our Solano runs (and locally on firefox).
This is because it was attaching a file that was not there. This change fixes the test
and makes the attachment method smarter so it will fail if it tries attaching a non-existent
file.
Add test file used for format validation.
Merge pull request #4591 from edx/benp/fix-attach-file

Fix false positive test in cms acceptance, harden attach_file helper.