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.
Yes. There is a testcase that is attaching this filename, but the file does not exist. The point of that testcase is different (it's testing for extension name), but rather than making the attach_file method more complicated, I'm adding this file so an actual one is attached.

The affected testcase for that file is in cms/djangoapps/contenstore/features/video_editor.feature (scenario 13).