Try this (just to confirm): once you load the detail view of the product reload the page and try the widget again.
I have detected that the first load of the page does not work most times but reloading works always. I think it has something to do with the old jquery library we are using and I am trying to organize a big change to be able to prove my theory
as i said : it's heratic, sometimes it works , sometime no, i have tested with different computers (windows , Mac, ) and different browsers
you have to reload the page twice or 3 times to works
looking at the chrome console log, the dropzone.js is loaded but not working
We have released a version 5.8 which has eliminated prototype. We are only using jquery now and the dropzone plugin seems to be working correctly on this new version.