Please help us help you and fill where relevant:
Your LimeSurvey version: 5
Own server or LimeSurvey hosting: own
Survey theme/template: default
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i would like to store files (from users and from the file manager too) not on the hosting server, but on a S3 bucket, for scalability purposes.

how could i accomplish this?
plugins?
extending the system with custom modules?
changing core modules? 
what about the views / UI? 
what are the pieces that touch the storage functionality?

i really appreciate your help! see ya around, and happy hacking.