Fix/reset dropzone on upload

Small bugfix to prepare for the "sort images" functionality.

Currently we're showing existing images below a dropzone. Inside of the dropzone are only images which are about to be uploaded, when save button is clicked. However right now, the dropzone content is not reset/cleared, when the upload was successful.

P.S.: Yes, this is not pretty. I'm open for other ideas, but I don't want to lift the internal dropzone state to the parent component.