Lack of 'send_stream_request' method.

[dijit]

Hi, I'm trying to upload images as part of my use of errbot on mattermost; I have the same bot listening on telegram, slack and mattermost so the plugins I write tend to uncover interop problems.

Based on the information in README.md you would assume that there is no upload functionality in mattermost (falling back to webhooks for cards) but I can see that this isn't the case.

Here is the equivalent function in the slack backend:

https://github.com/errbotio/errbot/blob/master/errbot/backends/slack.py#L695