https://github.com/elabs/refile

Refile is a modern file upload library for Ruby applications. It is simple, yet powerful.

Features:

• Configurable backends, file system, S3, etc...
• Convenient integration with ORMs
• On the fly manipulation of images and other files
• Streaming IO for fast and memory friendly uploads
• Works across form redisplays, i.e. when validations fail, even on S3
• Effortless direct uploads, even to S3