I'm so sorry that I am new subscriber of your channel and I don't know how to inspect the console.log(blob). 

thank you for your kindness David. I am not sure if this is the correct answer but, the log just shows this:

"undefined" on console.log(blob)

[Log] undefined (admin-116a425f6e4133270456.js, line 289)

Hi David! YES, I solved it!!

The issue was on DigitalOcean that it does not accept localhost:3000,

Response to preflight request doesn't pass access control check

This is how I solved.

1. create cors.xml
2. parse cors.xml to DigitalOcean Sapces through s3cmd by "s3cmd setcors cors.xml s3://{bucket_name}"

s3cmd installation:
https://www.digitalocean.com/docs/spaces/resources/s3cmd/

Your dropzone_controller.js code is the most simple and comprehensive, and it's a great starter compare to others in my opinion!

Thank you so much for your help David!  

☒

Thank you for your kindly providing the direction too! 

Can I also request you for the StimulusJS and Datatable.net tutorial?