upload and serve file to amazon s3 with sinatra [improved] reference: http://www.millwoodonline.co.uk/blog/upload-to-amazon-s3-with-sinatra

db-migrate-0001_create_table_s3_uploads

class CreateTableS3Uploads < ActiveRecord::Migration
  def up
    create_table :s3_uploads do |t|
      t. :url
      t.timestamps
    end
  end
  def down
    drop_table :s3_uploads
  end
end

form.html

<form action='/upload' method='post' accept-charset="utf-8" enctype="multipart/form-data">
  <label for='content_file'>Image</label>
  <input type="file" name="content[file]" id="content_file" />
  <button type='submit'>Save</button>
</form>

models-s3_upload.rb

class S3Upload < ActiveRecord::Base
end

routes-api.rb

def upload(filename, file)
    bucket = 'bucket_name'
    AWS::S3::Base.establish_connection!(
      :access_key_id     => ENV['ACCESS_KEY_ID'],
      :secret_access_key => ENV['SECRET_ACCESS_KEY']
    )
    AWS::S3::S3Object.store(
      filename,
      open(file.path),
      bucket
    )
    policy = AWS::S3::S3Object.acl(filename, bucket)
    policy.grants = [ AWS::S3::ACL::Grant.grant(:public_read) ]
    AWS::S3::S3Object.acl(filename, bucket, policy)

    # return url
    return AWS::S3::S3Object.url_for(filename, bucket, :authenticated => false)
  end
  
post '/upload' do
  uploaded_url = upload(params[:content]['file'][:filename], params[:content]['file'][:tempfile])
  # store the url somewhere
  S3Upload.create!(url: uploaded_url)
  redirect back
end