I'm making a Javascript app that collects the contents of a TinyMCE editor and sends it all to the server in a single post. I'm trying to allow the user to upload images into the editor, but I can't figure out how to do it without submitting the images at the time they're uploaded to a server-side image handler. I need a way to submit all images and text in one post.

I don't know a lot about handling images, but my thinking is that I can accomplish this by collecting the images as base64 strings and sending them along with the rest of the html input. First question: Is my line of thinking correct?

Second question: How do I accomplish this? I've managed to encode an uploaded image as base 64, but the TinyMCE editor image upload dialog box gets stuck open as if it's still uploading even after I already have the base 64 string.

index.html:

tinymce.init({
  selector: '.texteditor',
  plugins: 'autoresize code hr image link lists media searchreplace table charmap anchor help',
  images_upload_url: 'scripts/image_upload.js',
  images_upload_handler: function () {
    console.log("heres the image_upload_handler"); //This never fires
    upload_image();
  },
  automatic_uploads: true,
});

image_upload.js:

function upload_image(event) {

    const dropzone = document.getElementsByClassName('tox-dropzone')[0];
    var fileInput = dropzone.getElementsByTagName('input')[0];
    var files = fileInput.files;
    var file = files[0];
    console.log("File " + file.name + " is " + file.size + " bytes in size");
    getBase64(file);
}

function getBase64(file) {
    var reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = function () {
        console.log(reader.result);  //This successfully prints the base 64 string, but the editor's dialog box won't close and the image never appears in the editor
    };
    reader.onerror = function (error) {
        console.log('Error: ', error);
    };
}

When I upload an image using the editor, the result is this:

The console shows the image encoded as base 64, but the dialog box won't close. What do I need to do differently? Vanilla Javascript, please.