This is something I was trying to implement myself. I am surprised it can be done with just an s3 bucket. I was messing with API Gateways, Lambda functions and DynamoDB tables to support the s3 bucket. It didn't occur to me to implement it client side.
I might have stuck a bit too much to the lfs test server implementation. https://github.com/git-lfs/lfs-test-server
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