Rails direct upload to Amazon S3
Try a new Gem called CarrierWaveDirect it allows you to upload files direct to S3 using a html form and easily move the image processing into a background process
Not sure about whether you can modify it easily to only upload one file at a time, but this gem works very well for me. It is based on one of Ryan Bates' Railscast:
Try looking into carrierwave https://github.com/jnicklas/carrierwave (supports s3)Multi file uploads with carrierwave and uploadify http://blog.assimov.net/post/4306595758/multi-file-upload-with-uploadify-and-carrierwave-on