Function appendAttachmentToMessage

appendAttachmentToMessage(
    program: Program,
    message: MessageSpan,
    attachment: Attachment,
): Program
Adds an attachment to an existing message.
Parameters
- program: Program
  Program to edit.
- message: MessageSpan
  Target message span, as returned by findMessages.
- attachment: Attachment
  Attachment to add.
Returns Program
A new program; the input is left untouched.