import requests
from requests_toolbelt.multipart.encoder import MultipartEncoder

m = MultipartEncoder({'roomId': 'Y2lzY2.....',
                      'text': 'example attached',
                      'files': ('example.png', open('example.png', 'rb'),
                      'image/png')})

r = requests.post('https://webexapis.com/v1/messages', data=m,
                  headers={'Authorization': 'Bearer ACCESS_TOKEN',
                  'Content-Type': m.content_type})

print r.text