class Aws::S3::PresignedPost

def with(field_name, value)
  fvar = '${filename}'
  if index = value.rindex(fvar)
    if index + fvar.size == value.size
      @fields[field_name] = value
      starts_with(field_name, value[0,index])
    else
      msg = "${filename} only supported at the end of #{field_name}"
      raise ArgumentError, msg
    end
  else
    @fields[field_name] = value.to_s
    @conditions << { field_name => value.to_s }
  end
  self
end