Attach images to Markdown cells via notebook command.

[kolibril13]

Similar to #15615
I've got a Jupyter Widget, I would love to export an image from it to the next markdown cell.
I already managed to create a new Markdown cell using ipylab and anywidget, the last part missing is the command to attach the image to the attachments setting in the Notebook JSON metadata like this:

"attachments": {
    "cute_dog.jpg": {
     "image/jpeg": "/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAUDBAQEAwUEBAQFBQUGBwwIBwcHBw8LCwkMEQ8SEh

I've already had a look in the notebook commandsList, but could not find a function like add-attachments.

It would be great if there was one!

For reference, here is the widget that creates markdown cells:

import anywidget
from traitlets import Int, observe

class CounterWidget(anywidget.AnyWidget):
    _esm = """
    export function render({ model, el }) {
      let button = document.createElement("button");
      button.innerHTML = `count is ${model.get("value")}`;
      button.addEventListener("click", () => {
        model.set("value", model.get("value") + 1);
        model.save_changes();
      });
      model.on("change:value", () => {
        button.innerHTML = `count is ${model.get("value")}`;
      });
      el.appendChild(button);
    }
    """

    value = Int(0).tag(sync=True)

    @observe("value")
    def _observe_count(self, change):
        text = f"'Hello world, count is {change.new}' And here's the image ![cute_dog.jpg](attachment:cute_dog.jpg)"
        from ipylab import JupyterFrontEnd
        app = JupyterFrontEnd()
        # see all commands here for reference https://github.com/jupyterlab/jupyterlab/blob/main/galata/test/documentation/commands.test.ts-snapshots/commandsList-documentation-linux.json#L992
        app.commands.execute('notebook:insert-cell-below')
        app.commands.execute('notebook:replace-selection', { 'text': text})
        app.commands.execute('notebook:change-cell-to-markdown')
        app.commands.execute('notebook:render-all-markdown')

CounterWidget()

Screen.Recording.2024-01-07.at.19.02.15.mov