I used the S3 template to successfully implement an upload + download feature for my app. It works wonderfully, except for .xlsx files for some reason. I'm unable to actually open the files as Excel errors out saying "This file is corrupt and cannot be opened". I had a problem with .csv files as well.
.png, .pdf all seem to download without issue.
If I download the same .xlsx or .csv files straight from the S3 browser on AWS, it downloads without issue and is complete corruption-free.