Communicating these issues only over twitter is also not at all acceptable. Plenty of people don't use or rarely use twitter, such that it should not be considered the primary communication channel.

So while yes, College Board is allowing students to retake the tests, anyone affected should be refunded at least partially, if not fully, for having their time wasted. College Board should have had their backend reject invalid formats, not just hang. The result of attempting to upload an invalid format image should have been a 'whoops, please use a different format', but that's not what happened.