In the 2nd tab (Settings) in the widget, is defined wich kind of documents are allowed. default this are only image files.
If you make that field empty all file types should be allowed, and then at least the upload should succeed.
Does the user have ‘create’ rights on the entity ConfigFile? And your Dropzone should be in a dataview of SerialItem I think if I'm reading your domain model and config of the widget correctly.