s3_presigned_post.rb

require 'aws-sdk'

aws = AWS.config(access_key_id: ENV['AWS_ACCESS_KEY'], secret_access_key: ENV['AWS_SECRET_KEY'])
s3 = AWS::S3.new
bucket = s3.buckets[ARGV[0]]

form = bucket.objects[ARGV[1]].presigned_post

hidden_inputs = form.fields.map do |(name, value)|
  %(<input type="hidden" name="#{name}" value="#{value}" />)
end

puts <<-END
<form action="#{form.url}" method="post" enctype="multipart/form-data">
  #{hidden_inputs.join}
  <input type="file" name="file" />
  <input type="submit" value="upload">
</form>
END