Currently, i restricted my user to only upload images. When the user mistakenly select files that are of other type, such as .pdf .stl, the whole upload icon (which as a plus sign on it) will get disappeared after the user has selected the wrong file type. And the comment cannot be published too.
I have tested this on your demo site, it is the same scenario.
This issue will be fixed in the next update.
When i try to update the plugin with my reviewed licence, it said the files are not able to be upgraded.