It happened to me many times that I clicked Attach local file button and forgot to click Add button to really attach it. I thought it's already attached. But it was only selected/loaded but not attached to the support case.
I suggest to change Attach local file button's label to one of the following:
*Select local file
*Choose local file
*Load local file
And Add button's label change to one of:
because this button actually attaches the file to the support case.