A message was sent to a channel
{
"type": "message",
"subtype": "file_mention",
"ts": "1358877455.000010",
"text": "<@cal> mentioned a file: <https:...7.png|7.png>",
"file": {...},
"user": "U2147483697"
}
A file_mention message is sent when a file is mentioned in a channel,
group or direct message.
The file property contains a file object. The user
property contains the User ID of the user that mentioned the file (which may
differ from the user that uploaded the file).
When an event occurs, we will send an HTTP POST request to your Request URL. The outer payload is uniform across all Events API methods, and contains fields like team_id and event. Learn more