image_preview_controller.js

// app/javascript/controllers/image_preview_controller.js

import ApplicationController from "./application_controller";

export default class extends ApplicationController {
  
  static targets = ["imagePreview"];
  
  ...
  
  renderImagePreview(file) {
    const formData = new FormData();
    formData.append("image", file);

    fetch("/convert_heic", {
      method: "post",
      body: formData
    }).then(response => {
      response.blob().then(imgBlob => {
        let blobURL = URL.createObjectURL(imgBlob);
        this.imagePreviewTarget.insertAdjacentHTML("beforeend", `<img src="${blobURL}">`);
      }).catch(e => {
        // logic to show alert flash msg
      })
    })
  }
}