Verifying transactions with csv attachments - message contract

MessageContract.kt

class MessageContract : Contract {

  interface Commands : CommandData {
    class Send(attachmentId: AttachmentId) : CommandWithAttachmentId(attachmentId), Commands
  }

  abstract class CommandWithAttachmentId(val attachmentId: AttachmentId) : CommandData {
    override fun equals(other: Any?) = other?.javaClass == javaClass
    override fun hashCode() = javaClass.name.hashCode()
  }
}