How to store an uploaded file for later processing on Heroku?

I'm writing a PHP app that will process an uploaded file. A ZIP that contains a few CSVs, images, etc. The process requires user input if a warning/error arises while processing the file and the same file should be available to be re-processed later. On a normal server, I's use the filesystem to store the file, then keep the path on my DB.

However, in Heroku I can't do this. I'm using AWS S3 to store other file uploads. Should I store these ones too on S3 and each time I need them, download them to temp dir, process them, upload back and delete the local copy? Or is there a way to process the file while on AWS S3? Maybe mount the S3 bucket?