Mattermost-server: Add plugin API for UploadFile method

[jasonblais]

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The method UploadFile(data []byte, channelId string, filename string) (*FileUploadResponse, *Response) is available in model/client4.go, the Go REST API driver for Mattermost, but does not have an equivalent in the plugin API.

Not having this method in the plugin API limits what plugins can do. Hence, we would like to add an equivalent plugin API for it.

Read about adding to the plugin API here https://developers.mattermost.com/contribute/server/plugins/. Most of these methods should have fairly simple equivalents in the app package, making them trivial to implement.

You'll want to look at these files:

• https://github.com/mattermost/mattermost-server/blob/master/plugin/api.go
• https://github.com/mattermost/mattermost-server/blob/master/app/plugin_api.go

If you have any questions or would like feedback, feel free to ask in the Developer Toolkit channel on our Mattermost community instance.

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This ticket is part of a larger campaign for adding plugin API methods. See here: https://github.com/mattermost/mattermost-server/issues/9458

[hanzei]

I would like to work on this. This is harder as expected. I will leave this ticket for someone else.

[chclaus]

I will give it a try