Slack APIのfiles.uploadで画像を投稿するやつ

upload.py

# -*- coding: utf-8 -*-
import requests

url = 'https://slack.com/api/'
token = 'Slack Bot API Token'

def upload(filename, image, channels, filetype='jpg'):
    joind_channels = ','.join(channels)
    arguments = {
        'token': token,
        'title': filename,
        'channels': joind_channels
    }
    res = requests.post(url + 'files.upload', params=arguments, files={'file':image})
    print(res.json())

if __name__ == '__main__':
    with open('mazaiosamu.jpg', 'rb') as file:
        upload('mazaiosamu.jpg', file.read(), ['test'], 'jpg')