You can create a plugin e.g.

define(["tinymce"], function (tinymce) {
  tinymce.PluginManager.add("dnd-events", function (editor) {

    // Native-ish DOM events (TinyMCE forwards many of these)
    editor.on("dragover drop dragstart dragend", function (e) {
      // Your logic here
      // e.preventDefault(); // only if you really mean to override default behavior
      console.log("TinyMCE DnD event:", e.type, e);
    });

    // You can also use editor.getBody() to add DOM listeners if needed:
    // editor.on('init', () => editor.getBody().addEventListener('drop', ...));
  });
});

And register it in CMS 12 

services.Configure<TinyMceConfiguration>(config =>
{
    config.Default()
        .AddExternalPlugin("dnd-events", "/alloy/plugins/dnd-events.js")
        .AddPlugin("dnd-events");
});

This doesn't connect the .Net events to TinyMCE  if that what you wanted.

Optimizely already ships an epi-dnd-processor plugin that’s required for dragging Optimizely content (blocks/pages) into TinyMCE. So if your goal is to react to those drops, your custom plugin should listen but be careful about preventing default or stopping propagation, because you can break the built-in drop behavior.

Hi Eric

Many thanks for your help. I think I can do something with this approach. What I'm trying to solve is the problem of editors dragging images from the media gadget and losing the alt text in the process. Just having some way to react to this should give me a hook in.

Cheers,

Daniel

That might work. 

However we "usually" fetch the alt-text from the media item in the media display template.