Add attachment: error alert popup on "empty.png" does not show up

When you add a attachment with 1.) a illegal extension or 2.) with a too big size, then a alert pops up and displays the reason.

But if you add "empty.png" the error alert does not show up. Although a error-message appears when you try this with deactivated javascript.

This should be the reason, why the travis-test does not get this error.

So the ajax error alert has to be fixed.