MattermostはSlackのクローンであるが、incomingWebhookへのポストだけは、どうも違いがあるようだ。
以下にSlack/Mattermostそれぞれでの実例を示す。
まずmattermost

# mattermost

import requests

incomWebHook = 'http://192.168.10.1/hooks/xxxxx'
msg = 'test message'
icon_url = 'http://192.168.10.10/graphix/robot.png'
username = 'bot'

def sendMsg(incomWebHook, icon_url, username, msg):
    payload = {
            'text': msg,
            'icon_url': icon_url,
            'username': username}

    r = requests.post(
        incomWebHook,
        json=payload)

    return r

r = sendMsg(incomWebHook, icon_url, username, msg)

Slack

# slack
import requests

incomWebHook ="https://slack.com/api/chat.postMessage"
msg = 'test message'
username = 'bot'
token = "xoxb-xxxx-xxxx"

def sendMsg(incomWebHook, token, username, msg):
    payload_dic = {
            "token": token
            "type": "message",
            "channel": "#general",
            "text": msg,
            "username": username,
            "icon_emoji": ":bird:",
            }

    headers = {
            'Content-type': 'application/json',
            'Accept': 'text/plain'
            }

    r = requests.post(
            incomWebHook,
            params=payload_dic,
            headers=headers)

    return r

r = sendMsg(incomWebHook, token, username, msg)