In order for your application to access the AWS credentials for signing upload requests, they will need to be added as configuration variables in Heroku:
heroku config:set AWS_ACCESS_KEY_ID=xxx AWS_SECRET_ACCESS_KEY=yyy
Adding config vars and restarting app... done, v21
AWS_ACCESS_KEY_ID => xxx
AWS_SECRET_ACCESS_KEY => yyy
In addition to the AWS access credentials, set your target S3 bucket’s name:
heroku config:set S3_BUCKET=zzz
Adding config vars and restarting app... done, v21
S3_BUCKET => zzz