Slash command attachment not working

Hello,

I'm using slash commands to create a BOT, but I can't seem to get the attachments working.
As response I have this json string:

{
   "response_type": "in_channel",
   "text": "Test",
   "attachments": [
        {
            "image_url": "http://url.to/my_image.png"
        }
    ]
}

But only the text "Test" is appearing in the channel, but everything inside the "attachments" is not.
While for the mattermost_bot this does work.

What could be wrong?