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file_comment message event

A comment was added to a file

Compatibility: RTM

    "type": "message",
    "subtype": "file_comment",
    "ts": "1361482916.000003",
    "text": "<@cal> commented on a file: ...",
    "file": {},
    "comment": {}

A file_comment message is sent when a new comment is added to a file shared into a channel, group or direct message.