text/markdown/basic html support

[fonsp]

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[leonhandreke]

Do you have an opinion how this should work on the UI side?

Here is an idea: Have a textbox under the image upload thing. Once you input text, a send button appears next to it. But: you can also still select a picture, which in this case will not be sent immediately but only when the send button is pressed. Behavior if no text is entered remains the same. I'm not sure this is a good idea though, since it will sort of build an expectation that images also have text, and I quite like that this is not the case right now.