class AWS::S3::PresignedPost

def field_name(option_name)
  case option_name
  when :expires_header
    "Expires"
  when :server_side_encryption
    "x-amz-server-side-encryption"
  when :key, "Key", :policy, "Policy"
    option_name.to_s.downcase
  when :acl, :success_action_redirect, :success_action_status
    option_name.to_s
  else
    # e.g. Cache-Control from cache_control
    field_name = option_name.to_s.tr("_", "-").
      gsub(/-(.)/) { |m| m.upcase }
    field_name[0,1] = field_name[0,1].upcase
    field_name
  end
end