Message.getJSON() does not contain all properties

[Simon-Laux]

some file properties are missing - we should add them.
and after that cleanup the MessageType in desktop that is mostly there as an workaround around the message.getJSON not providing all values.

[Simon-Laux]

current definition in desktop:

export type MessageType = JsonMessage & {
  sender: JsonContact
  setupCodeBegin?: string
  file_mime: string | null
  file_bytes: number | null
  file_name: string | null
}

so this issue is still valid?