allow to send files in Extra via /api/message

[adbenitez]

feel free to take over the PR if something is wrong or to tweak, I am a golang noob

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[42wim]

LGTM, thanks!

[tehsu]

can this be used to send images?

[adbenitez]

any file you want

[LawrenceWence]

@adbenitez any chance that you would provide a working example? I can't figure our how to properly submit a png/jpeg via extra data

[UnkwUsr]

@LawrenceWence you have to send next json to api endpoint just as you sending text:

"Extra" : {
    "file": [
        {
            "Data": "here should be base64 encoded file",
            "Name": "filename"
            "Comment": "comment, just text"
        }
    ]
}

But there is also problem that current latest release version (1.26.0) is broken (at least for me, with forwarding to telegram), so you might need to build manually master branch version of matterbridge. #2182