Issue
You need to store temporary files on the Dyno
Resolution
Application processes have full access to the available, unused space on the mounted /app disc, allowing your application to write gigabytes of temporary data files. To find approximately how much space is available for your current Dyno type, run the CLI command heroku run "df -h" --size=standard-1x -a APP_NAME, and check the value for the volume mounted at /app. Different Dyno types might have different size discs, so it's important that you check with the correct Dyno size
Please note: Due to the Dynos ephemeral filesystem, any files written to the disc will be permanently destroyed when the Dyno is restarted or cycled. To ensure your files persist between restarts, we recommend using a third party file storage service.