Missing file comment in incoming SlackFile

[aabiryukov]

Additional to issue #9

When SlackConnector deserializing file data it loses file comment. Slack response has file comment in json field: file.initial_comment.comment.
It is major metadata for the file message.

Example of file message with comment:

As result, is not possible to get file comment from incoming SlackFile.

[aabiryukov]

Surpise from slack.
Yesterday Slack changed the format of message with image. Now SlackConnecter has empty file info :(
News (without details): https://get.slack.help/hc/en-us/articles/115004846068-Slack-updates-and-changes#july-2018

New message format has multiple files:

{
. . .
  "text": "USER MESSAGE",
  "files": [
    {
. . .
    }
  ],
}

[Workshop2]

https://api.slack.com/changelog/2018-05-file-threads-soon-tread

[Workshop2]

file.initial_comment.comment isn't a valid property. You can find the message the file was uploaded with under the SlackMessage text

[Workshop2]

File -> Files released in 4.2.71